When I rebuilt the 318 in my old D-150, it had been getting a consistent
14.5 mpg on the highway and about 11 mpg around town.
That was with the 2 bbl Carter emissions carb, dual plane EGR intake and all the smog equipment. In the rebuild, I kept only the PCV. I used a 1972 calibrated Carter carb and the single 2-bbl intake from a 318 72 Dart.
In theory, because both main jets are now feeding all cylinders instead of each jet feeding half of the cylinders, my gas mileage should have dropped. to about 10 or 12 on the highway. With duals, my son gets 26 mpg highway
Instead, I gained 10 mpg on the highway (24 mpg) and 8 mpg in town (19 mpg) on 89 octane fuel.
More fuel doesn't mean less mileage if you have more air to mix with it.