I once got 18 mpg on the trip to the ATHS National Show in Baltimore a few years ago on a 55 mph trip with a stock 235 and stock 3.90 rear end.
20 mpg with a 4.11 rear end and a 216 sounds rear nice.
I am now getting about 15 mpg with a dual-carb, dual-exhaust 261 and a 3.55 rear end. This was on a 230 mile trip from Macungie PA at 60-65 mph with a calibrated speedometer and a very heavy passing-foot.
