Yeah, I get horrid gas mileage with my Colorado. I completely destroyed what little aerodynamics it had when I ditched the stock bumper, lifted it, and put that aluminum contractor cap (sits 4" proud of the cab roof) with a cab-over rack on it.
Then I put 33" mudders on.
I get 12-13 mpg on the highway in the winter. Gets up closer to 15 in summer.
It got 21 when I bought it.
I have a 72 mile round trip commute. Buying gas sucks.
My uncle just bought a new crew cab Chevy 2500 with the newly designed Duramax diesel... He gets 19-20mpg on the highway in a truck twice the size of mine.