I don't know about 20mpg.
That might be a
bit much to ask of a V-8 with a carb in a car that weighs over 3,600lbs and has no OD tranny. 15 overall is probably a more obtainable goal. Don't try to extrapolate the mileage numbers of a modern fuel injected computer controlled car with a 4 speed OD tranny to what you can get with a 35 YO car that turns 1,000 more RPM at highway speed.
The thing to do to avoid run-on is to
leave the car in gear when you turn it off. The 318 is not normally plauged with after-run unless it has many miles and lots of carbon build up.
Besides the obvious ones the areas to work on for better mileage are>
1) A better exhaust, like a decent dual exhaust with a crossover pipe.
2) As much initial ignition advance as possible without detonation and make sure the vac advance in the distributor is working like it's supposed to. Having that unhooked is death to mileage.
3) A carb as lean as possible. You might even be able to find a NOS 2 bbl carb on E-bay for cheap. I'd look for the newest model I could find for a 318. I would NOT use a 340 Intake on a 318 because you'll have a horrific port mis-match that will kill your mileage.
4) Make sure the front end is aligned correctly.
5) Use synthetic oil
6) Don't haul 100 lbs of crap around in your trunk.
7) Might also wanna see which intake manifold your motor has. Some of the old A motors came with a weird single plane 2 bbl manifold. I think a dual plane is always the best choice for TQ and mileage. It's easy to find one in the boneyard.
Also realise that mileage figures are like performance estimates. You can usually subtract 4 or 5 mpg (or mph) from whatever most of your buddies tell you they get with theirs. And also remember you use regular and they can't.
