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.
526 cubes of angry wedge, pushbutton shifted, 9 passenger killer!
