Gas Mileage query Holley 318

If a 360 with an xe 268H and 750DP carb can get close to 20 mpg with 3.23 gear, you are a bit off on the tune up.

Holley carbs are notoriously rich EVERYWHERE in the curve.

Get an AFR meter and go to work tuning the brass in that carb. It takes more than swapping out jets alone.