On my 74 Duster360, I took out the 3.55 rear end and swapped in a 2.45 rear end (complete, drum to drum) before going to Carlisle, back in 1999. The RPM at 70mph went down to 2500-2600. Was able to get 23mpg all day long at 70mph. With the 3.55, mileage was 18-19mpg . . . big difference on long trips. The 360 had more than enough torque to spin the right wheel (open rear) with the 2.45 gears.
2500-2600 rpm with 2.45 @ 70? that's more like a 2.93 gear.
235/60/14 tire . . . with that 2.45, that was what the rpm was - 2500-2600.
Gotta think that all the other variables (intake, carb, exhaust, cam, convertor, tranny, etc) factor into all this. Of course each engine needs to be in "it's sweet spot" where it is happy . . .
