my dad has a 07 duramax truck, crew cab short bed all stock, we live in seattle and he gets around 20mpg on average, well his other house is in montana, and over their he gets about 14mpg, its about a mile up in elevation, is their anything that could be causing this, maybe a computer reprogram for the elevation, or does it need a chip and intake to help this out

thanks

Going up and over the mountain passes plays a big role. More demand = more fuel.

yup, those hills eat up fuel especially when towing.

we could get technical and say that it could be the o2 diffrence at the higher elevation.