Standard were the 318 and the Slant 6. The former should average about 16-17, I reckon. The Slant 6 should reach the 20s on the highway if you’re lucky, but it’s a heavy car with a small engine needing all it has to push it.

The 383-2 is really not much worse off than the 318, but has a lot more torque. I think averaging 15-16 would make sense. With a 4bbl. you could expect 13-14, maybe less.

The hi-po engines - the 440-4, 440-6, and Hemi - will impact mileage due to the high performance nature of the cars, if not the gearing. The 440-4 can probably run as efficiently as a 383-4, but that’s with a light foot and higher gearing. Otherwise, I would expect to average 12. With the Six Pack, I think you’d be lucky to average the same. And I know an old Consumer Guide test that got 6mpg with the Hemi, although that’s probably an extreme - it’s probably an 8-12 mpg car.