The 225 in my duster got fair mileage. Some people have luck with a mopar getting good mileage. Mine never really did. I've known alot of people over the years that had a 318 in both cars and trucks and the mileage isn't there.
If you can't get good mpg from a 318, I stand by my first reaction. "You must not know how to tune or drive." If you want to compare compacts or small engines, to a 318 what is your point. I worked on all brands of cars. Mopars were always at the top for power efficiency, and reliability. And I worked on cars when Pontiacs had Pontiac engines, Buicks had Buick engines, Oldsmobiles had Oldsmobile engines, not the cheap Chevrolet junk engines.