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Author Topic: Best Single MPG Mod?  (Read 12946 times)
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Oleksy
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« on: 08/06/11 05:33 AM »

What's the best single mod to improve gas mileage?

I was going to order a cold air intake, but after doing a lot of surfing, this doesn't look like the best mod to improve gas mileage.  Would a Superchip programmer be better?
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« Reply #1 on: 08/06/11 05:59 AM »

Buy a 4-cylinder car! Sorry, I couldn't resist.

From my experience you won't improve your mileage much along the way when you start modding. If I was going to pick one mod it would probably be a programming suite like HPTuners or EFI Live. They aren't cheap but they give you complete control over adjusting your pcm and tcm. If you aren't up to lots of reading and learning neither one of these programs is for you. Next on the list, I guess I would try either  dyno tune or mail order tune, making sure I explained to the tuner that my goals were greater mpg. Next would be a canned programmer but they are limited in the amount of adjustments you can make.

Realistically, you need to do something to the pcm, the intake, and the exhaust to see the full effects. You have to be able to pull more air in and remove air out more efficiently. You can do a drop-in K&N filter for the stock airbox pretty cheaply and it will gain you probably 80% of the benefits of a CAI.

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« Reply #2 on: 08/06/11 10:26 AM »

Best single mod in my opinion is a competent, tailored tune. Second, depending on your existing shoes, may well be a carefully chosen, new set of tires.
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« Reply #3 on: 08/06/11 11:29 AM »

As easy as it sounds, and VERY hard to do for a lot of people..  Go the speed limit..  It makes a HUGE difference, that and like i was brught up before the size of you rims etc makes a big difference.  I went back to my stock setup and gained between 2 and 3 MPG..

Good luck.. 
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« Reply #4 on: 08/06/11 12:17 PM »

As easy as it sounds, and VERY hard to do for a lot of people..  Go the speed limit..  It makes a HUGE difference, that and like i was brught up before the size of you rims etc makes a big difference.  I went back to my stock setup and gained between 2 and 3 MPG..

Good luck.. 

Good concept, but as you point out, sometimes hard to do. And even then....truck gets great mileage at around 65, not quite as good at 70, really starts to fall off at/after 75. Twenty miles of my 25 mile one way commute is in free flowing, 75 mph speed limit, traffic. Eighty is the norm and under 75 you'll get run over/are clogging up the flow.
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« Reply #5 on: 08/06/11 01:12 PM »

Sorry to say, but reality is the stock system is probably the best for MPG.  Yes, programmers, etc can get slightly better MPG, but I don't think you'ld really earn your money back.  I have a CAI, Dual Exhaust, and programmer and my MPG is way WORSE than stock.  I got the Edge Evolution Programmer and it acts as a gauge cluster while running.  I can tell you that avg MPG is MUCH better at 55 than 75.  But I know it's hard to stay off the pedal when you hear that throaty roar!
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« Reply #6 on: 08/06/11 10:48 PM »

brick under gas pedal....
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« Reply #7 on: 08/07/11 12:52 AM »

brick under gas pedal....
And an egg on top of the gas pedal 
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« Reply #8 on: 08/07/11 02:11 AM »

Rig up a SAIL panel... heheheuhuhuheheh...
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« Reply #9 on: 08/07/11 03:26 AM »

Sorry to say, but reality is the stock system is probably the best for MPG.  Yes, programmers, etc can get slightly better MPG, but I don't think you'ld really earn your money back.  I have a CAI, Dual Exhaust, and programmer and my MPG is way WORSE than stock.  I got the Edge Evolution Programmer and it acts as a gauge cluster while running.  I can tell you that avg MPG is MUCH better at 55 than 75.  But I know it's hard to stay off the pedal when you hear that throaty roar!

With something like a Predator programmer or a tuning suite like HPTuners it's pretty easy to fix. If you're really serious you could set the speed limiter to, say, 70 mph. You couldn't go any faster than that. It's probably not a good idea but if you really wanted to you could.
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« Reply #10 on: 08/07/11 03:40 AM »

True.  But I think that's the point of programmers - I don't WANT to limit it.
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« Reply #11 on: 08/07/11 03:50 AM »

Thanks for the input.  I do tend to drive like an old lady, and coast a lot, but I'll still like to get better MPG.

If the CAI negatively impact MPG, would a drop in result in lost mpg too?  I'd be willing to go the drop in route ...

Also, I'll look at a programer.
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« Reply #12 on: 08/07/11 04:14 AM »

I have to ask what kind of mileage are you getting now.
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« Reply #13 on: 08/07/11 10:10 PM »

Good concept, but as you point out, sometimes hard to do. And even then....truck gets great mileage at around 65, not quite as good at 70, really starts to fall off at/after 75. Twenty miles of my 25 mile one way commute is in free flowing, 75 mph speed limit, traffic. Eighty is the norm and under 75 you'll get run over/are clogging up the flow.
But its much harder to run over an avalanche..  Thumbs up!
 Same here on the LONG commute as I am at 33 miles..  But I also knew that keeping the av was going to create a higher gas bill each month..
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« Reply #14 on: 08/07/11 11:11 PM »

Buy another car and only use Avy when you need it...
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« Reply #15 on: 08/08/11 01:24 AM »

Buy another car and only use Avy when you need it...
Looked into that. Way too expensive.  Even with a 100% increase in mileage (30 mpg), your second car/truck/motorcycle is going to run more because of insurance and general maintenance.  I seriously thought of this for my 20-mile commute, but dropped it because it just wasn't worth it.  After my dad passed away, I even drove his sub-compact Chrysler Sebring for a few months. It was a PITA to try and coordinate when I'd need the AV (for baseball crap for my kids) and not.  After a month or so, I decided to simply sell it.  The extra gas mileage wasn't worth the insurance cost or feeling like I was a sardine.

I vote for the brick idea!
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« Reply #16 on: 08/08/11 08:01 AM »

Thanks for the input.  I do tend to drive like an old lady, and coast a lot, but I'll still like to get better MPG.

If the CAI negatively impact MPG, would a drop in result in lost mpg too?  I'd be willing to go the drop in route ...

Also, I'll look at a programer.

A dyno tune with the emphasis on economy will be yyour best bet. A programer will do the same but a dyno tune has more parameters to adjust.

Avalon
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« Reply #17 on: 08/09/11 01:08 PM »

Right now, with a mix of driving, I'm getting 15.9.  I have made a few trips pulling my light boat, which lowered the mileage.

Of course, I'd like to get a little more....
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« Reply #18 on: 08/19/11 05:20 AM »

So I was really interested in increasing my MPG and PowerBlockTV had a special they did on a mid 90 Dodge Ram 1500 with lifted tires.  I have started to do much of what they tested and it does work.  I don't know all of them but this is what I have done and intend on doing in the future to increase the MPG of my 03 Avalanche 1500.  When I started the quest for better gas milage I was high 10's low 11's on a weekly basis.  Without changing my driving I am now at high 15's and that is not highway driving.  Highway I have been upwards of 20 MPG. 

1. BIGGEST PAYOFF- buy a computer programmer.  I have a hypertec and it increased my milage 2-3 mpg.  It does cost but if you look at it like this, after you re-coupe the savings of the cost of gas, you will then be saving money.  Mine cost around $300 and I bought it a few months ago.

2. CAI- Just with that I was able to get another 0.5 MPG out of it, don't need to buy an expensive AFE, I have had one of those on a 05 GMC and it was great, bought a cheap on on other cars... just a good.  The one I have on the AVE is a ebay special.  I love it.

3. Make sure your vehicle is tuned up and maintenance is done. i.e. tire pressure, good oil....

TO BE DONE:
1.  Change out the mechanical cooling fan for an electric cooling fan.  Cost is about $100, only a few wires and then you get around 2 MPG. 

2. Make sure that there isn't any extra weight on the vehicle that isn't needed.  i.e. loading straps, oversized spare tire, junk, toolboxes that aren't needed, additional hitches... just make sure you don't have a bunch of crap that doesn't need to be riding around costing you MPG.

3.  Driving style is a big deal, PowerBlock TV installed a vacuum gauge in order to help control the gas pedal, shutting off your vehicle doesn't help t stop lights, and definitely don't shift to neutral going down hill.

4
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« Reply #19 on: 08/19/11 08:29 AM »

those are all good tips.

What if you buy a 60 MPG Toyota Prius and put that in the bed?
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« Reply #20 on: 09/18/11 12:43 PM »

 Why not use neutral down long hills? Out of my drive, I can coast for 6.3 miles. Adjusting RPMs before engaging tranny, what does it hurt?
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« Reply #21 on: 09/18/11 11:52 PM »

Why not use neutral down long hills? Out of my drive, I can coast for 6.3 miles. Adjusting RPMs before engaging tranny, what does it hurt?
transmission.
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« Reply #22 on: 09/19/11 01:41 PM »

I tried shell v power lowest octane without realize it before my research and got better mileage. I viewed this site and found ethanol free gas and got better milelage with considerable city driving. 2-3 mpg estimated,I typically get less than 16 with very much city driving and driving highway 55 in Missouri I get 16 or less. Last tank 17.4 including that city driving. http://www.pure-gas.org/extensions/map.html

I filled up today with shell v power 93 octane and I will let you know how that goes. $.24 more per gallon. It was not marked no ethanol.

I have a rough idol and I hope it clears that up, on other sites there have been some great increase with the 4 cylinder engines.

Here is a site that help you track your mileage   http://www.Fuelly.com/car/
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« Reply #23 on: 09/20/11 01:41 PM »

18.5 on the way back from Cape Girardeau mo. 93 octane not sure about the ethanol in the gas or not. I will finish the tank tomorrow or thursday to see what the DIC  calculates. Highway 70 is usually by best mileage unless the head wind is high.
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« Reply #24 on: 09/20/11 09:33 PM »

gas weighs a lot especially a full tank if you don't drive far every day just keep the tank half full or even just above a quarter tank, it  got me a little better mileage
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« Reply #25 on: 09/20/11 11:10 PM »

gas weighs a lot especially a full tank if you don't drive far every day just keep the tank half full or even just above a quarter tank, it  got me a little better mileage
gas weighs about 6-7lbs per gallon I forget actual number but is less then water @ 8 lbs per gallon...

so 15 gallons which is half a tank = using 7 lbs = 105 lbs...

Which for the 6000 pound truck is not very much... I bet it effects the mpg less then 0.01%

the best way tpo get better mpg... is to take off very very very slowly.... from stops.. as that is where the majority of the fuel is wastd getting this pig of a truck moving....
 
also anything ovr 50 mph is wasting huge amounts of fuel fighting the wind..
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« Reply #26 on: 09/21/11 01:10 AM »

gas weighs about 6-7lbs per gallon I forget actual number but is less then water @ 8 lbs per gallon...

so 15 gallons which is half a tank = using 7 lbs = 105 lbs...
Yes, gas is roughly 6lbs / gallon. I know jet fuel is a little more at 7 lbs./gal.  (I used to be a sub-contractor working at an Air Force base, so we'd calculate these things.)

Which for the 6000 pound truck is not very much... I bet it effects the mpg less then 0.01%
My thoughts, exactly.  6 lbs * 26 gal = 156 lbs. 

If this were a Yugo, I'm sure that 156 lbs. would have a drastic effect. On an Avalanche - not so much.


the best way tpo get better mpg... is to take off very very very slowly.... from stops.. as that is where the majority of the fuel is wastd getting this pig of a truck moving....
I remember Sperry mentioning that he tried to keep his truck under 2000 rpm. Not sure how he did it where he lived but I do the same. For most of my drive, I can keep the truck under 2000 rpm, even on starts. That really helps out.


also anything ovr 50 mph is wasting huge amounts of fuel fighting the wind..
But it sure helps when you have an 18-wheeler on your tail!
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« Reply #27 on: 09/22/11 12:15 PM »

Don't mean to  Thread hijacked! but it's related to your question. The AV has now quit down shifting on hills and some were fairly small and milage is improving with 93 octane still not sure about the ethanol.
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« Reply #28 on: 09/23/11 12:06 AM »

Tune it for 91 or 93 octane gas (which ever is available) and always fill it with that. The MPGs and added power will justify the higher priced fuel.
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« Reply #29 on: 09/23/11 04:38 AM »

Loss of mileage is for me related to oil loss again  banghead
So I wake up to a check engine light this morning stop by auto zone number 7 misfire. Same thing I had last time. I have already scheduled a test for this next week. I plan to make a poll here to see who has had this is who hasn't. What milage apprx did it start? Has it reoccurred? If you don't have the is what do you typically dive city or highway and how many miles?
If I can't get a resolution a lawyer will be spoken to.
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« Reply #30 on: 09/23/11 06:17 AM »

gas weighs a lot especially a full tank if you don't drive far every day just keep the tank half full or even just above a quarter tank, it  got me a little better mileage

That's intersting. I've heard that having the truck less then 1/2 full will cause it to use more gas, which as I'm typing this makes utterly no sense to me now. I just got 16.8 from my 2004 with some rather agressive driving. I'm a teacher and I've been really agitated after school Can't  Beating the students, so I took it out on my gas pedal and really  Drive! this last tank. I also had to put 31 gallons into it and was terrified I wouldn't make it a gas station, but I did.
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« Reply #31 on: 10/07/11 02:27 PM »

Tune it for 91 or 93 octane gas (which ever is available) and always fill it with that. The MPGs and added power will justify the higher priced fuel.

So I am planning on the diablo that just went up for sale on this site. If I tune to 93 (only 93 or 87 available) will there be issues when only 91 is available?
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« Reply #32 on: 12/02/12 02:09 PM »

Why not use neutral down long hills? Out of my drive, I can coast for 6.3 miles. Adjusting RPMs before engaging tranny, what does it hurt?

This actually uses more gas than if you just let it coast in gear...just saying.
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« Reply #33 on: 02/13/13 04:34 PM »

I get better mileage when I use premium fuel. So I always try to put premium in it.
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« Reply #34 on: 02/13/13 06:04 PM »

I saw on top gear that coasting at speeds uses almost no fuel in modern cars, because the spinning of the tranny keeps it running while using less fuel. Don't know how true it is but I thought it was interesting.
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« Reply #35 on: 03/08/13 04:27 AM »

That's intersting. I've heard that having the truck less then 1/2 full will cause it to use more gas, which as I'm typing this makes utterly no sense to me now. I just got 16.8 from my 2004 with some rather agressive driving. I'm a teacher and I've been really agitated after school Can't  Beating the students, so I took it out on my gas pedal and really  Drive! this last tank. I also had to put 31 gallons into it and was terrified I wouldn't make it a gas station, but I did.

As an engineer, I know where this comes from.... And in older cars, yes this is somewhat true. In an older car, the fuel systems were very, I mean VERY basic. They were actually exactly what you would imagine in your head right now. A simple fuel tank with a spout, or filler hose, that led up to an accessible hole where fuel could be added. In this gas tank was a basic fuel pump, which was somewhat easily accessible for a reason (always had to be replaced). Things were even worse before EFI came about. Anyways, back on topic. Fuel tends to evaporate relatively quick. Therefore, this claim has to do with the rate the fuel would actually evaporate from the tank whether it was actually in use, or just sitting over night. A smaller amount of fuel (i.e. 1/4 tank would obviously evaporate faster than a full tank, due to surface area etc). New cars have a lot more advanced fuel systems. They eliminated nearly all the evaporation issues (from the little flap on the filler spout to advanced gas caps, circulation hoses .. the list goes on).  But thats how that theory began and people have tended to continue believing it.  Trust me, you waste more money by making more frequent stops to refuel due to only filling up halfway, rather than filling up and not having to stop as often.
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« Reply #36 on: 03/08/13 04:38 AM »

I apologize, I just re-read what I wrote and it probably doesn't make sense to some.   People used to think that a way to reduce loss due to evaporation would be to keep the tank full, which reduced the amount (volume) of space above the liquid (gasoline) in the tank.

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« Reply #37 on: 03/08/13 05:53 AM »

I apologize, I just re-read what I wrote and it probably doesn't make sense to some.   People used to think that a way to reduce loss due to evaporation would be to keep the tank full, which reduced the amount (volume) of space above the liquid (gasoline) in the tank.



I'm kind of an old school engineer too and I remember when we kept the tank full not because of evaporation but because of condensation. 

The theory was that when the tank had air space in it, the air contains water (humidity) and if the temperature changes then the condensation (translate water) would sink to the bottom of the tank and then on into the engine. the other theory was that it would also rust the tank bottom (I don't see that oxidizing effect happening when there is no oxygen under the fluid however).

There still may be some truth in this condensation theory today but with the complexity of the modern vehicles, I would think this would far down the list of things that could go wrong.

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« Reply #38 on: 03/08/13 07:37 AM »

i know if im on my "budget tanks" where i am trying to save fuel i will adjust my pedals all the way forward... flooring it isnt near the same that way it cuts the acceleration down greatly
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« Reply #39 on: 03/08/13 09:00 AM »

If you can draft off of a big rig, it saves a ton of gas. I'm not saying its safe but it does work. And the truck driver probably wont like it Beating
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« Reply #40 on: 03/08/13 09:15 AM »

Best MOD I would say is a custom tune.
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« Reply #41 on: 03/08/13 10:14 AM »

I saw on top gear that coasting at speeds uses almost no fuel in modern cars, because the spinning of the tranny keeps it running while using less fuel. Don't know how true it is but I thought it was interesting.

The way the maps are written, high vacuum, no load, idle-ish TPS readings=no fuel. You end up with a giant air exchanger. They keep the converter locked in D and OD to maintain the pumping action.
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« Reply #42 on: 03/31/13 05:01 PM »

Remove or reduce unnecessary weight.
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« Reply #43 on: 03/31/13 06:00 PM »

You could try the UPS method - Adjust your routes so that you make alot of right turns and avoid stopping at traffic signals and left hand turns.

Seriously, there is no mod or upgrade that is going to improve your mpg drastically. Many of us are happy at 15-16 city and 19-21 hwy. Anything above that is a fantasy.  Depending on your wallet, driving habits and type of daily commute will determine your mpg. Do your research, upgrade the air filter to a K&N, upgrade your plugs to iridium, clean your MAF sensor, buy a tuner, change tires to city/hwy rated tires, drop the spare (keep the slime kit in the tool box), use fuel system cleaner with each tank fill up, change your oil every 3k, cut your vehicle warm-up time, etc. http://[b]It's 7200 lb aerodynamic Brick. Built for those who love the option of fun and versatile daily hauler with room for 5 or full size drywall panels. Excellent towing capcity for our weekend toys and enough to enjoy a day off-roading.[/b]

The wife and kids love it because of the styling, versatility and modifications for a day at the beach, in mountains, antique shopping, moving friends or a night on the town. Inaddition, they don't have to worry about filling up the tank, I do.

Enjoy the vehicle for what it was designed for and not it's limitations.
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« Reply #44 on: 02/22/14 11:02 AM »

Great thread.
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« Reply #45 on: 02/22/14 02:19 PM »

Big Pimpin... you bumped a thread from march of '13. The rest of your posts are only a few words long also and dont really state anything of use and now you have an Avy for sale thread. Way to go!
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« Reply #46 on: 02/23/14 07:41 PM »

#1 way to improve MPG is to instal skinny light weight wheels. 

Guaranteed double digit percentage increase if you go skinny enough. And it would look so cool  Thumbs up!
 
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« Reply #47 on: 02/24/14 03:14 AM »

Realistically if you are worried about gas mileage on a full sized truck that weighs almost 1000 lbs more than a standard pickup truck you have the wrong vehicle...

Having said that unless you get your PCM dyno tuned and maybe spend some money getting your engine running more efficiently you REALLY can't do anything much from stock to make your truck get better mileage. The tuners that are mentioned really are guesses and are going to be of very limited value. Going to a premium gas does absolutely nothing for you unless your engine is changing its timing to take advantage of it. And realistically you should NOT be getting better gas mileage from higher octane gas because it burns faster. It MAY help with in town driving but it is more for performance and should have a negative affect on your long range mileage.

CAI... Stock already is...

You can try a pseudo forced air intake but it does minimal actual improvement and generally may degrade performance if you can't keep the air cooled off. (Ram air)

If you are worried about the cost of fuel you could look into going to Propane or CNG either of which is not only far cheaper than Gasoline but also help to extend the life of your engine and are zero emissions. Actually much cleaner long term than an electric car that generally depends on coal generated fuel to produce power. Company I talked to told me to expect a 3 year return on a system in my truck if I drive 12,000 miles a year and gas is $4 a gallon. So a little longer if gasoline is less. At the time the CNG was available for the equivalence of $2 a gallon. They also mentioned that the CNG actually got better mileage due to the fact the engine stayed cleaner and it is a gas so it doesn't have an issue mixing.

Otherwise the only thing you can do is get good road tires and maintain your vehicle replacing worn components. GM did a pretty bang up job trying to get as much efficiency as possible out of the truck from the factory so most anything you do will have a negative effect even if initially it seems to be a positive effect since the PCM will learn around it.
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« Reply #48 on: 02/24/14 08:15 AM »

Best mod for MPG is a tune...right foot tune, that is

I've read a lot about never coming to a complete stop...may be tough for the city guys but for me in the suburbs if I see a light change red from a ways away I can try to anticipate when it's going to change back to green and keep it above 15-20MPH or so and thus have to accelerate less.  The worst MPG is when you're accelerating from a stop so in theory this should help a lot

And as previously mentioned keeping the RPM below 2000 and limiting highway speed (tough for me, ~50 miles each way) does in fact help.  Pumping up tire pressure helps too.

Anybody tried an octane booster or a fuel injector cleaner?
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« Reply #49 on: 02/24/14 11:26 AM »

Why not use neutral down long hills? Out of my drive, I can coast for 6.3 miles. Adjusting RPMs before engaging tranny, what does it hurt?

You use less gas going down hill in drive with foot off pedal (fuel injectors are turned off) than putting into neutral (fuel injectors are flowing fuel to keep engine running).

Best single mod for MPG? - Keep off the brake unless really needed.  I gain speed going down hill & lose a little speed going uphill, I let off the gas when I see a stop light ahead avoiding the brake as much as possible.  I see using the brake as lost gas mileage.

I'm a big fan of K&N drop in filters with over 500,000 miles of use.  I little oil goes a long way, too many over oil their filters which causes problems.

I've also found I get more miles/$ by running a 93 tune with premium fuel than a 87 tune with regular fuel if I keep my foot out of it.

I enjoy my jackrabbit starts & get to speed quickly which is also where I get my best MPG.  Some take way to long to get to their best fuel efficiency speed which actually hurts their MPG.

My typical mixed MPG is 17-18 & hwy is 22-26 depending on elevation changes.
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« Reply #50 on: 02/24/14 11:36 AM »

You use less gas going down hill in drive with foot off pedal (fuel injectors are turned off) than putting into neutral (fuel injectors are flowing fuel to keep engine running).

Injectors are not turned off with foot off of pedal.  Largely for emissions purposes the engine is still running in closed loop operation and still looking for the same fuel/air ratio.  The computers have provisions for tuning in a more fuel saving fuel/air ratio in these situations which is not used for US cars.  Additionally the stock tune in my 03 would increase airflow over stock levels in these situations which causes an increase in fuel usage with the locked fuel/air ratio.
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« Reply #51 on: 05/17/14 06:23 AM »

Well I have a 03 z71 and I do good to get 13.6 mpg. 285/75/17's ,true dual straight pipe , k&n CAI , new plugs and wires, 114000 miles and the oil doesn't hardly even change color between changes. I would give anything to get 16-17 mpg. Would y'all suggest I get a programmer or chip to match the other mods? Or would that make it burn more?
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« Reply #52 on: 05/17/14 07:54 AM »

It's been said for years around this site the best MPG mod is your driving style. You need to drive like an old geezer and avoid stop and go driving and idling as much as possible.
An AV is 6,000 lbs. and takes a lot of gas to get it moving and stopped. It also has the aerodynamics of a brick and doing things like lifting it and putting on big rims or wide tires can affect airflow or rolling resistance. Despite the manufacturer's clams of improved gas mileage I have read of a decrease in MPG with CAI and that may be due to the driver stomping on the accelerator to hear the intake noise. (Same could be said for exhaust mods too I guess.)
I know gas prices are going up but I can't see spending hundreds of dollars on tuners or other items to get one or two MPG increase, you would have to burn lots of gas to get it to pay off. I can see someone getting a tuner for performance gains though if they are looking for HP gains.
Just my .02 but the same argument could be used for getting a Magnacharger supercharger setup for a AV since it would help a heavy vehicle get up to speed quicker and would not burn more gas at cruise over stock, so if driven properly could in theory help mpg. The trick is, could a person have the ability to use in the "efficient" manner and not stomp on it to feel the effect or hear the sound? 
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« Reply #53 on: 05/17/14 10:58 AM »

As an engineer, I know where this comes from.... And in older cars, yes this is somewhat true. In an older car, the fuel systems were very, I mean VERY basic. They were actually exactly what you would imagine in your head right now. A simple fuel tank with a spout, or filler hose, that led up to an accessible hole where fuel could be added. In this gas tank was a basic fuel pump, which was somewhat easily accessible for a reason (always had to be replaced). Things were even worse before EFI came about. Anyways, back on topic. Fuel tends to evaporate relatively quick. Therefore, this claim has to do with the rate the fuel would actually evaporate from the tank whether it was actually in use, or just sitting over night. A smaller amount of fuel (i.e. 1/4 tank would obviously evaporate faster than a full tank, due to surface area etc). New cars have a lot more advanced fuel systems. They eliminated nearly all the evaporation issues (from the little flap on the filler spout to advanced gas caps, circulation hoses .. the list goes on).  But thats how that theory began and people have tended to continue believing it.  Trust me, you waste more money by making more frequent stops to refuel due to only filling up halfway, rather than filling up and not having to stop as often.

As I remember the theory it was that a half filled tank gave better mileage than a full tank......and it goes further back than that to the days where there was no air pollution controls and tanks were fully vented to the atmosphere. the only openings in the gas tank was the filler opening at the top and the gas supply line coming out the lowest part of the gas tank......fuel was gravity fed to the mechanical diaphragm fuel pump located low on the engine and powered by a lever action off an extra lobe on the camshaft.....no pressure regulator, fuel feed controlled by a shutoff valve on the float in the carburetor. These 50/60s era sometimes would "vapor lock" on a hot day when the fuel pump to carb line became too hot and heated the gas forming an air pocket in the line , had to either cool down the line or bleed the air off at the input fitting on the carb....later improvements included a return line to the gas tank to equalize pressure and prevent a dead ended lockup. Psychological effect or real?, but those cars always ran better and got worse mileage on a full tank of gas......likely because it was a gravity feed system and the higher the gas level in the tank, the higher the incoming pressure of the gas being fed to the pump.....in fact many times on a full throttle acceleration the reserve in the carb bowl would go dry and all you had was gas as fast as it could be supplied by the pump with its tongue hanging out, so if you wanted better acceleration, on to a 2 bbl., then 4 bbl. then 3x2 bbl. But today, none of this applies so  you'll get the same gas mileage no matter what the level of gas in the tank.
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« Reply #54 on: 05/17/14 11:18 AM »

  My best MPG mod was swapping in a new cam.  I gained 20% fuel economy!
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« Reply #55 on: 06/06/14 07:48 PM »

I had a 91 mercury capri convertible, 5 speed/4 cyl that I got used to taking out of gear all the time when I could. A lot of hills here in SW VA, could get as much as 36-37 mpg with that car! Rust underneath(MI car) finally made me retire it at 340,000. I used to put my Avy in neutral, could get upper 18's with mixed driving. Decided to stop after reading input on this topic. Don't wish to tear it up Cheesy
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« Reply #56 on: 06/28/14 02:40 PM »

  My best MPG mod was swapping in a new cam.  I gained 20% fuel economy!

Cam>6 speed manual Huh
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« Reply #57 on: 07/01/14 05:22 AM »

Cam>6 speed manual Huh

Yep.  That 6-speed is huge and the rotating mass isn't helping the MPG.  The cam gave me an instant 20% increase in MPG.  The trans is mainly for heavy duty towing...to the tune of 35,000 pounds worth.
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« Reply #58 on: 07/01/14 08:45 AM »

That's surprising, but I remember jacking that zf6 up into my f350 and it was huge.  It looked like Ford pulled it out of a small semi truck and stuffed it in there.

BTW,  they're super tough,  but they can wear out if you give one to the wrong driver (like a stubborn step dad that thinks he can drive anything)
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« Reply #59 on: 07/05/14 08:47 AM »

Yep...the ZF6 went in some one-ton pickup trucks, but mostly the heavy duty stuff like the Ford F-550's and dump trucks.  They weren't specific as far as Ford/Chevy etc.  There were some slight differences like gear ratios, but dimensionally the same size.

P.S.  Someone needs to fix this site's spell checker.  It is annoying getting flagged when I spell something incorrectly, and go online looking it up only to find that I spelled the word correctly.
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