It depends entirely on the engine load and the engine's efficiency map. So there's no real answer, but in general as rpm increases, internal friction goes up and economy goes down. So, IN GENERAL, lowering the numerical gear ratio should increase mileage.

But here's a true story of my '64Dog. It's a 318 Poly stocker D100 and it came with 3.91 gears. Rear tires were 32-11.50 15, 636 revolutions per mile. I used to drive long distances and it would get a maximum of 15mpg. Much of that time was spent on Interstates at 80. Calculating that out that's 3320rpm at 80.

Then, I swapped rears as I had a donor truck and the '64 axle was leaking onto the brakes. The donor truck had a 318 and 727, 3.23 gears. New rpm,2740 at 80.It's like overdrive, right? To my dismay the next trip fuel mileage was same as before, 15mpg.

Changing the gear ratio that much really made for a better road truck but around town it lacked the snap it had with the 3.91s.

R.