Best mileage I've recorded is 20.5, that was with me, my teenage son, a mountain bike, and 75 lbs of tools in the car. It was also before I rebuilt the carb, and dialed in the rest of the tune...
I'd be willing to bet that I'd get 22 mpg now with the car similarly loaded. 25 wouldn't be difficult with some real care paid to tune-up, tire pressures, etc.
And this is in a wagon, which has to weigh a few hundred pounds more than a 2-door sedan.
I think 25 mpg should be obtainable by most /6 A-bodies, 30 would be pushing it, and 30+ should be reserved for the best LSD trips.
