I'm getting 20mpg with my 225 /6. All I've really done is replace the carb with a stock Holley 1945 (reman) and swap to GM HEI electronic ignition. That and a good tune up and fine tuning the timing and carb.
And that's around the best they will get give or take about 5MPG. There are those that say "my slant gets 30 blah blah blah" but either they are stretching the truth or they don't know the correct way to measure mileage. There was really only one package that got that kind of mileage from a slant. Those were the "light" cars. The Feather Duster and the Dart Light. They came with aluminum and lightened body panels and 4 speed OD transmissions. The automatic equipped cars got around 25.
