I am using a 1991 non LSD convertable 5-speed in my N/A S4 swapped B2200, and I an wanting to change my 5th gear ratio, as I think its too low, and lugging the engine at highway speeds.

At 60MPH, I am running at 2000RPM in 5th. The truck has a 3.909 final drive ratio, which means, I have a 0.69 5th gear. I have an 86 FC tranny, which from what I read online, is a 0.711 final drive, and I also have the old transmission from my truck, which is the same series as the RX7 tranny, its 5th gear is 0.89, which I think will be too high of an RPM.

What are your opinions? Should I put the 0.89 5th in my truck, or the 0.711, or stay with my 0.69?

I would estimate the weight of the truck a 2700-2800lbs, but no where near as aerodynamic as an RX7. I do plan on using this truck as a truck still though.