Gas mileage is affected by many factors - the largest factor is rolling resistance. I have owned wicked cars that got good gas milage
One is particular: E Body 416 six pak with 4sp OD and 355's
16 mpg going to and from Carlisle, back in forth to Watkins Glen etc. Top speed 140+ ET 12.20 to 12.80 with 11.90 trap speeds 400+hp at the rear wheels

Place car on level pavement in neutral
Can you push it backwards without effort??
Probably not. This is the test, if you can't do this there will be no gas mileage.

Synthetic fluid in the diff, run level 1/4 inch down.
Rear wheel bearings greased with synthetic grease, end play 0.005-0.007
Front wheel bearings in synthetic grease, bearing preload about 50 inch pounds.
Proper wheel alignment
Manual Trans? Passon gear lube
Auto Trans? Mopar ATF4 or better yet Amzoil synthetic

Engine oil, 10-40 or 10-30 synthetic, 5-30 if your engine will tolerate it

Proper engine tune up and distributor curve - here is the other major mileage factor. Lots of timing advance under part throttle needed.

K&N air filter

60% distilled water, 40% antifreeze, bottle of coolant enhancer.

195 degree thermostat

External Trans cooler if AT with internal radiator connection bypassed