Originally Posted by John Strenk
Displacement has nothing to do with with MPG. Weight, rolling resistance, speed are more important than displacement.
Yes and no, but mostly no. You've obviously never owned a 454 in a light vehicle like a vega or nova. They still get 10-12mpg in a chassis that gets 20mpg with a 200ci straight six or little 140 4 cyl.
Weight only comes into play when you start working a small engine hard. At that point added displacement has less of a mpg penalty, but ultimately displacement is the #1 factor. It's less of a penalty.
However, back to the OP's question, I doubt there's more than 1 or 2 mpg difference on the highway with big tires and bad aerodynamics. The little six will be working hard and your foot will be way down, the 6 will be taking in maximum air per revolution with the throttle open that far. The V8 with a barely open throttle now has the opportunity to achieve similar mileage to the 6. The place the six will trump the v8 is around town when it's not being worked hard and has a lot of idling time.
If you're actually concerned with mileage I'd shoot for a V8 around 300 inches. It's a good balance of torque and power and mileage. AMC purists don't wanna hear it but a ford 289/302 or chevy 283/305 will get better mileage than a 304 due to rotating mass and head design.
Since mileage relates to load on the engine, and a lifted big tire truck has higher load on the engine, don't trust the EPA figures above. Higher load is going to make the 6 and 8 mileage closer together.
There is no straight answer. You may think you got one but you're talking modified vehicles and there's too many variables to get the simple answer you're seeking.
Also, when you hear people talking about getting 6mpg from a 360, understand that their engine has jacked up carburetion and timing. I get 6mpg with a carbureted 454 towing the equivalent of 4 jeeps, going 70mph, turning 3100rpm. There is no way a properly tuned small block should get 6 on the highway unless it's towing a house.
4 out of 5 backyard builders don't know how to set up carburetors or distributors and cost themselves huge mileage numbers.