My 318 Poly got a maximum of 16mpg with 3.23 gears and 32" tall rear tires. That's with the carb leaned out, which means down jetted until it has lean surge then one jet number higher.

I replaced it with an '86 318 roller motor. I home ported the 302 heads, swapped in a 360 /5.9Mag camshaft, added the cast iron EGR 360 manifold, adapted the Poly's carb to it. So same carb not changed and same exhaust system with stock low perf manifolds piped back to the exact same exhaust system. This is a 100+ K engine. First tank was 22mpg! That's in a '64 D100 long sweptline.

That chamber is magic.

R.