Max MPG- slant vs 318/340

Got 20 mpg average coming back from Minnesota in july, and that was fully loaded with parts and other crap that I brought along!. My setup is a 5speed with 3.25 gears, late 80s roller cam block, cam is stock, roller rockers, autolite carb, which made a big difference in performance and fuel economy, stock manifolds, HEI ignition and eddy intake. Did get on it a lot and cruised it at 75 mph most of the way but if I was driving 65, and not loaded up and wasn't getting on it a lot I can see getting 25 mpg.