[Lugnutz65]

I have talked to a lot of owners who want to improve their fuel economy.
The popular choices in no particular order are:
1. Keep the 3 speed and swap in a 3.08 rear differential
2. 700R4 AT with a 3.42 or 3.73 rear differential.
3. T5 5 speed MT with a 0.86 OD gear and a 3.42 rear differential
4. T5 5 speed MT with a 0.72 OD gear and a 3.73 rear differential
5. Tremec makes "bulletproof" 5 and 6 speeds but very $$$.
Fuel injection will help improve MPG as well.

Most importantly, match the rear differential with the transmission OD gear.