I just recently bought a used 2010 GMC Sierra 1500 ext. cab 4X4 with the 6 speed/5.3 combo. In the city, I'm getting 17-18.5L/100 km. That's a far cry from the sticker which stated it gets 14.4 L/100 km. The previous owner has the original Bridgestone Dueler tires on the truck; 18" in size. Would moving to 17" wheels improve mpg? I would have to change the rims too to lower the truck as well. Also, do Michelin tires do better with mpg than the Bridgestone ones would?