A heavy foot makes for bad mileage..I've never actually seen a full time 4wd chevy truck that got better than 6-8 mpg no matter how you drive it nor what the condition of the motor is/was..one of my co-workers has a 77 2wd 3/4 ton w/ 4.11 gears and an sm465 and a freshly rebuilt mild 350 and has never seen better than 8 mpg, and thats with a light foot..