Gas mileage 225 Slant 6

but the slant 6 never got 30 plus.
I agree. They were hard to get 18-20.Now under the right circumstances yes they can do better.
For the average Joe, not us that tune to the nth-degree . Also terrain plays a part.
You live in the flat lands ? Mountains ? maybe just hilly ? Our 70 Dart 225 auto on a good day was 14 16 at most. That was in the hills of southern Pennsylvania .