It's your gear ratio that's killing your mileage.
4:10 is mighty steep unless your towing a trailer or pulling stumps.
If you can change to a 3:50 ratio or something close, I'll bet your mileage will be in the 17-20 range @ 55 mph!
I have a modified 292 bored .40 with 3-2's, cam, headers and an .075 OD with 3:55 gears and I get 18-20mpg @ 65mph at 1950 rpm's.
RapRap
1940 ChoppedChevy Coupe
