Gas Mileage query Holley 318

My Duster with a mild cam, Edelbrock Performer 318/360 intake, 750 DP with a stock 78 318 bottom end running through an A500 OD auto and a 4.10 geared 8-3/4 rear with 28" tires got me 20.6 mpg after tuning it for mileage. It maybe could have got a hair better but I quit messing with it. That was at steady 65-70 mph, 25-2600 rpm on a 300 mile round trip.

Cley