The 318 has never been known for good mileage. All else being the same what is the real difference between the mileage potential on the 318 and the 360?
What is your cars weight, trans, gear ratio & tire size in what year A body?
If you (wisely) ignore the factory tune up specs and give it the hot rod treatment in the distributor and run the crab a hair lean, you can do very well.
The mention of an OD transmission is a nice idea but it can be a bit of a PIA to install and get everything done that needs to be done. Though, once done it does offer a lower first gear to get going easier and quicker along with the OD ratio of .69. Gear ratios work out like this when in overdrive;
Gear ratio multiplied by overdrive ratio for the final drive ratio.
4.10 = 2.829 (2.83)
3.91 = 2.697 (2.70)
3.55 = 2.449 (2.45)
3.23 = 2.228 (2.23)
2.94 = 2.028 (2.03)
2.76 = 1.90
