one time, a million years ago in the mid 70's, i had a 74 dodge 4x4 pickup with a 318/727/3.91 gear 35" tires that got right around 5mpg pulling a super bed load of junk to the scrapper. it was tuned to the max, but to be fair, i had added a "super" bed made entirely of 1/8" plate steel, with 2" oak floor, and a 3" roll bar using 1/4" tube gas pipe. [and that was a real "treat" to bend !] it had 6"&8" C channel cross members, as well as a piece of 6" well casing between the rear frame horns. up front, there was a fabbed 2" push bar on top of some 4" well casing for a front bumper. to say this thing was heavy would be an understatement ! i built a 440 8.5cr with a torque/rv cam, TQ carb/factory hi stall converter, and swapped it for the 318. mileage went instantly to almost 14 ! the 318 was foot to the floor all the time, while the 440 was just loafing. i wished i never sold that truck. it could sure haul the loads, plus push/pull almost anything out of the road !