My 87 Diplomat was running a 318 with a 600 cfm Edelbrock carb, MP ignition, A833 and 2.94 SG rear. Got 25 mpg highway regularly, on regular.

No skinny tires though, which might have bumped the mileage a bit. Dead reliable, cheap and easy to fix.

A benchmark, FWIW.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.