Smallblock efficiency or How to have your cake and eat it too

BigBlockMopar said
Good topic.

I run a 11.3:1cr 360ci every day in my '73 Dart. 0.030" overbore. Propane fueled.
318 '302' heads, CompCams XE256h, converted to solid, KB-pistons w/balanced crank assembly.
A518-OD (no LU), 3.55:1 gear ratio in a 8-3/4" rear axle.
MegaSquirt ECU controlling ignition only for now. Spent months tuning to come up with a curve that works nicely for the engine.

The car doesn't get even near the mileage I had hoped for... 11-12mpg, mixed highway+street.
Highway only driving barely gets 13mpg.
I'm mostly blaming the A518 currently, and its most likely unmatched mid '60s stock stall convertor currently in the mix.

As you mentioned sounds like the converter may be an issue. I did a 340 build trw 10-1 pistons, 272/455 purple cam, performer intake, thermoquad, J heads 2.02 valves, and headers, 518 non lock up w/ stock rebuilt high stall 340 converter, 3.55 gears in a Challenger that would knock down 18-20mpg on the highway. I would use more gear or a smaller duration cam if I were to do it over.