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How accurate are the factory installed MPG computers

2013avalanche

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Mar 2, 2014
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138
I have checked my computer readout to manually doing it and it is always off 2-3MPG, always saying better than actual mileage. Its says 17-18MPG on computer but only getting 13MPG. anybody have same problem with the 2013 avalanche digital readout?
 
I think most all of them are that way. I have an 06 and it is never right either. I have another 2014 Toyota and it is 2-3-mpg off every time also.
 
They are only as accurate as their calibration values in the computer.  Obviously the calibration values aren't spot on - hence the use of the O2 sensors to close the loop and calculate fuel trims to get the O2 levels right.  Even if they then apply the fuel trims to better calculate the amount of fuel used they still can't account for the MAF calibration being off.
 
Mine's pretty close to right on in road trip, steady state driving. Otherwise maybe 7.5% optimistic....on average
 
Typically a gallon off on use but as much as 4 and then I suspected the gas station pumps. Also the pumps are aloud to be with in 5 percent at 5 gallons.
 
It also depends on other functions as well. You are only seeing the instantaneous MPG from the computer. It should be reasonably accurate for long distance trips or highway usage where there is little stop and go. But otherwise if you are a lead foot your MPG display will never be accurate because you really dump fuel when you stomp on it taking off from a light. But it is fun I know.. Just know it sucks the gas down when you do it.

Not to mention that when the truck starts up and it is cold it will run rich for a while to warm up the truck until the O2 sensors are hot enough to work. So if you do a lot of short trips or lots of stop and go traffic the MPG reading will be inaccurate for your type of driving. Not to mention it uses a time weighted average so if you stomp on it and get to speed quickly that won't show on the MPG displayed.
 
Geek alert in 3... 2...

You may be forgetting to account for fuel spent idling - especially if you remote start.

Think of it like this. If you top off your tank and drive 31 miles to your house and let the truck idle until it's out of gas - mathematically (traditional MPG calculation) - you got 1 mile per gallon (if you have a 31 gallon tank).

My phone app (and fuelly) say I'm currently getting 10.x MPG - but that is based off of gallons purchased and miles drove. It does not account for the 12 minute remote start warms ups four times a day. For the best accuracy, you need to know your trucks GPH (gallons per hour) fuel consumption at idle - and how long your truck spent idling (remote start/stop lights/construction/grid lock - traffic jams/waiting for wife to get out the bathroom so you can leave ... etc.).

Starting with a full tank of gas I drove 202 miles then topped off the tank with 24.11 gallons - thus my MPG would have been 8.38MPG. (202m / 24.11g = 8.38MPG)
What's missing is it took me 23 days to drive the 202 miles. Each day I remote started my truck at least 4 times - each time letting it run the full 12 minutes - at the end of the day my truck idled for 48 minutes warming up. My truck burns 0.4 GPH (or .006 GPM) at idle in Park (0.8+ at high idle)(per bulley dog tuner). Thus I burned 0.32 Gallons of gas warming up my truck each day or about 6.4 Gallons in those 23 days. To correct for this - in the true spirit of the MPG hoopla - we subtract the 6.4 gallons (spent idling) from the 24.11 (we just purchased) (because we where not actually putting on "miles" idling) then recalculate. 202m / 17.71g = 11.405 MPG. So - as you can see, there is a 3.025 MPG difference because of the gas spent idling warming up the truck.

A quick internet search says that the "average" time spent at an individual stoplight is about 1 minute. I seem to hit about 2 stoplights on my daily drive. I live by a busy street that takes about 1 minute to merge on to. So, 3 minutes burned (again) 4 times a day. So there's another ? gallon unaccounted for. I typically eat out about 1 once a day - with a 5 minute drive-thu wait - so another ? gallon. 202m / 16.46g = 12.272 MPG

Lets pretend you loose 1 gallon of fuel a month due to evaporation = 13.065 MPG.

As you can see, the more we account for spent fuel that is not adding mileage to the vehicle - the more accurate our MPG will be.

ALLLLLLL of that being said; I have no idea how the computers calculate mileage. They could very well monitor the VSS and "stop the clock" unless you're rolling.
 
A fwiw comment......My 2013 V6 Honda Accord is very accurate no matter how/how differently I drive it tank to tank. So, it can be an accurate function if done right.
 
I recently filled up, drove 4 hours on the interstate at 68 mph nearly the entire trip. I got off the interstate and filled up immediately. The display stated I was getting 19.8 mpg. When I calculated it, I only got 17.1 mpg. The exact same thing happened when I returned. I spent no time idling. It was all driving time.
 
In-car MPG computers are, or at least were, notoriously overzealous in their showings
 
IMO, the instant fuel mileage reading is to be used as a guideline only to assist the driver to maximize the amount of fuel being burned at any given moment.

The digital readout in our trucks reminds me of the old vacuum "Economy Gauge" that car manufacturers put in some cars back in the 1980's.

If you had one of those gauges, you learned to keep your foot out of the gas in an attempt to keep the gauge's needle in the "good" area.

If you had a light foot, the engine's vacuum stayed high and the gauge gave you a nice reading.

Of course, there were some that mashed the gas at every opportunity to see how long they could keep the gauge in the red.

One of my past cars was a 1980 Pontiac Turbo Trans AM.

On that car, Pontiac saw fit to install a "Turbo Charge" gauge on the hood scoop facing the driver.

As the turbo's boost increased, another one of the three indicator lights were illuminate, until all three were on at maximum boost.

I can't image that gimmick did much to increase the fuel economy of that series of cars.

Chevrolet Z-28's of the same time frame had an electrically operated set of air inlet doors that would open when you smashed the gas to the floor.

And let's not forget the Chevrolet Chevelle SS series and their pop-up air intake on the hood.

Dang, that was a nice walk down memory lane.

Now, what were we talking about?

(y)
 
You can compare it against a GPS unit. Fill up the tank, turn on the GPS unit. Document the mileage and fill it up again. That will give you what you need. Then have it recalibrated at the dealership.
 
I find mine is close when I take long trips.
 
I only did one tank, but my 2013 was within 0.1mpg (reading on the optimistic side).  I'd say that's pretty darn close.
 
I only check mine when taking trips and the Av's is consistently 1mpg better than when I calculate it.
 
Always a bit optimistic.

The giant spike was probably not quite a full fillup, and then the following fillup took more than it should have.

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Just did a 1000 mile test, computer vs my math. I got 17.4 with the computer and 17.5 with my math-cant get much closer than that.
 
Just got back from a 3,000 mile road trip with our 2013, reset the mpg reader at the start of our trip and got a best of 23 mpg between fillups ( a lot of 65 mph long highway miles ), by the time we got back it was reading a stong 20.5, and yes I was cross checking at each refill with my calculator, dash MPG computer only off at best a mile or two per 600 + miles per tank. Used 87 octane the whole way. Very happy with fuel mileage for our new Avalanche (y)
 
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