Max MPG- slant vs 318/340

The low cr 318 in my dart managed to get almost 19mpg on a 200mile highway trip just last week. Running on propane, tuned very lean.
The car has an A518 OD but 3.91 gears in the 8-3/4".

I'm fairly certain if the lame compression ratio in the 318 had 2 points more it could very well mpg into the 20's somewhere. On propane.
Gasoline should be able to get 1 or 2 points more perhaps.