if you want lower RPMs at 60mph you should put in the 3:66 gearset that come in 3.2L and 3.5L cars (non 300m special)
This is your answer. 3.89 is what comes with 300M Special's and 2.7L LH vehicle's, it's the go-too for torque and speed. Only other options would be Prowler gears and Prowler Pro Gears. But performance options.
If you run some numbers and know your math and find it will be in more of your favor swapping output shaft and differential shaft gears; then it is an option. Providing both gears are not identical in tooth count (excuse me if I never counted them LOL).
Pretty easy procedure, more so with the transmission OUT of the car.
- Raise the vehicle. Level is recommended since you likely don't want to be showered in extra transmission fluid. But consider this a good time for a new gasket and filter.
- Put in a jack to support the transmission, if you secure it under the oil pan, I suggest a wood block across the entire pan to ensure nothing gets dented.
- Remove the six bolts on each side of the support beam, which connects to the body and engine cradle. CAREFUL, they will shear as they tend to corrode, suggest putting anti-seize on everything you put together that is steel-on-steel.
- Remove the four or six bolts securing to the transmission. CAREFUL, they will shear if they are built up with oxidization.
- Remove the gear cover.
To remove gears, just rotate the fancy clip until it lines up with the gear splines and pulls off. Gears slide off very easily.
You'll need to visit Dodge and get some ATV transmission sealant for the cover when sealing it back up.
Expect to loose 2-4 liters of fluid as well. Use ATF-4 ONLY. If you reuse your old fluid via catching it by a container, I SERIOUSLY recommend a paint strainer in the funnel with a magnet. This way all dirt and debris is caught.
That said, I get 30-35 MPG with my Police 3.5 with ported heads, new injectors, 3.89 gearing and shift kit, on autostick. Mind you I'm driving terribly, well - by the books and it feels slow and awful.
In the famous words of Jeremy Clarkson... POOOWWWEEEERRR!