1973 Dart Swinger Gas mileage

I would be happy with 17, back in the day i had 70 ford f250 with a 460, 7mpg, 12 on the hwy.
If they fit you could get larger diameter tires. If you up your rear tire dia by about 10 percent that's the equivalent of going from 3.23 to 2.94. your speedo will be off so you wont be able to measure your milage.