My old 86 W250 with a 440, 4 spd, 285/75/16s and 4.10s got terrible mileage. Less than 8 mpg in most cases. However, I never expected it to get very good with a 850 double pumper on it. I didn't care what it got, just as long as it ran good. If you want mileage, you need to go diesel.