Torque + Fuel economy

I have two dodge trucks. 1984 D-100 slant six A-833od 3.91 SG rear. When I first got the truck, I towed my race car, from Florida to Clay City Ky. I did absolutly nothing to the truck, except change the oil between getting it and leaving. I got 13 mpg on the trip. I will say the truck has some upgrades by the previous owner. Non lean burn, and a super six intake/carb. I have never checked unloaded mileage, as I don't use it much (the clutch slips under heavy load).
My 88 D150 has a 360 /727 with a Q-jet carb. That truck gets 9-9.5 mpg empty, or pulling a race car.