The 318 has never been known for good mileage. All else being the same what is the real difference between the mileage potential on the 318 and the 360?
The 4" bore of the 360 has always proven to be more of a gas hog than the 318, although the 360 can get decent mileage under the right circumstances. With either engine, you still have a V8, so there's that.
