What Is Good Gas Mileage?

I was wondering what kind of gas mileage people are getting from performance modified 318 engines? Particularly anything with 318 block but 340/360 heads as well as cam upgrades and 4 barrel carbs? Have not had a chance to inspect more closely the engine of my 69 Dart, but the guy I bought it from said the mileage was not the best with his engine combination (318 block, 2.02 heads from a 360, Mopar purple cam, 4 barrel carb). Makes me wonder if all of the components were properly matched for peak performance. Any info on gas mileage would be appreciated.