All tests that I have seen dropping the tailgate hurt the mileage.

On the other hand, a cab-high aluminum bed topper seems to help the mileage.

Personally, the two largest gains I made on my own 1968:

(1) Switch from a dogmatic to a 4-speed (24 percent)
(2) Removal of the "bug screen along the front of the hood (12 percent)

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop