Gas mileage is a matter of Physics. "It takes so much gas to move a given weight over a given distance".I specialized in this field for years. The more efficient your engine is the better but light cars get better mileage than heavy ones. Having said that, your half ton could get lousy mileage also if everything isn't right.My 65 Chev. PU in original condition (283 2 barrel carb. & 3 spd.)got 10 MPG with 3:73 gears. Now it has a 400 H.P. 400 small block with an early Corvette close ration T-10 4 spd. & same 3:73 gears and gets 16 MPG. Honest!