I recently bought a 2005 Avalanche. The Average MPG readout says I am getting around 12.5 to the gallon. Most of my driving is city with maybe 10% highway. According to the VIN, I have 3.42 gearing and the tire size is stock. Is this average for the city? The original sticker says 14 city and 18 highway. I've checked the tire air pressure and it is correct. Tires are Bridgestone Dueler A/T Revo 2. The air filter is also brand new.