You are all right. Weight, axle ratio, how a vehicle is driven, how slippery it is all play into the equation...

A little practical experience. My neighbor had a 1980 Chevy 3/4 ton pick up on a 1 ton chassis. It's 350 was tired and not pulling his trailer as well as it use to. We installed a 454 with about 1 hp per cube.

Mileage in town with the 350 was 10-12. Mileage in town with the 454 was 13-14... Highway was even more dramatic. He got 13-15 with the 350 and 16 pulling the trailer with the 454.

Same mass, coefficient of drag, tires and driving style. The 454 just did the work easier. Of course he could have driven the truck harder, faster with the 454, but that wasn't the goal. So apples to apples, at least for him the 454 paid for itself in fuel savings.