3.91 good or bad

bmaudlin said

I'm thinking of running 3.91's in my 70 Swinger. Hooked up behind a TKO 5 speed, should make a very comfortable drive on the highway in 5th gear. Had a TKO in my 69 Camaro and it was a blast to drive, and actually increased the gas mileage by almost double on the highway--in town in lower gears was a different story though! Ben.

Click to expand...

O/D transmissions throw an entirely new factor into the equation.
With the .68 final drive in that TKO 5-speed (rear tire size & cruising speed dependant) 4.10:1 and even 4.56:1 would work well on the highway.



1:1 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,900 RPM @ 60 MPH
.68 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,000 RPM @ 60 MPH
.68 final drive + 4.30:1 gears + 27.1" tires (255/60R-15) = ~2,175 RPM @ 60 MPH


.68 final drive + 4.56:1 gears + 27.1" tires (255/60R-15) = ~2,300 RPM @ 60 MPH
1:1 final drive + 3.23:1 gears + 27.1" tires (255/60R-15) = ~2,400 RPM @ 60 MPH
1:1 final drive + 3.55:1 gears + 27.1" tires (255/60R-15) = ~2,640 RPM @ 60 MPH