440 hone re ring mild 220--226 duration cam, Qjet carb, 14degrees initial timing 36 total all in by 2800 12 degree vacuum advance for a total of 48 to 50 at cruise and 60 mph will get you 15ish mpg. The holly double pumper on a truck will just use fuel to use fuel. Yes you could tune it to do really well BUT can you tune yourself to be fuel efficient using it?.