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To get more performance out of your car you should definitely consider switching to a higher gear ratio. The question is, which one is right for you?

If you are considering changing your final drive ratio with a gear swap consider this:

The choice comes down to whether you're looking for pure acceleration or top speed. As with any choice it's a trade off. Use this rule of thumb:
  • The higher the gear ratio, the easier it is to get the vehicle moving and better accelerating, but lowers the top speed.
  • The opposite is true of lower gear ratios. The lower the gear ratio, the more difficult it is to get the vehicle moving, but the top speed will be higher.
With the exception of a few manual transmission (M5R2) Super Coupes, all 1994 - 1997 MN12s came with the 4R70W transmission. At the far end of the power train, common factory installed final drive ratios for MN12's and FN10's range from 2.73 to 3.31. See this thread for details.

3.27's are a great mild upgrade from 3.08s and they were even included on some MN12 cars from the factory. Check the "AX" code on the door jam sticker for your axle code to see where you're at now. Again, refer to this thread for details. 3.27's will get the car moving nicely with negligible fuel economy or top end sacrifices.

Moving up, 3.55's are an excellent choice for highway use giving the best balance between acceleration and top end. Plus, you usually don't need a tune with 3.55 gears and would be OK with just a speedometer gear swap.

At the high end, 3.73's are an even taller gear but are still relatively highway friendly. This is a popular choice and is as high as you would want to go on a vehicle that sees any amount of highway use. For more information see this thread for detailed feedback on 3.73's and how they affect performance of the MN12.

Gas mileage decreases with higher ratio gears will not be too drastic, depending on gearing, usually no more than 3-4 MPG. You will really only notice that on the highway and not in town unless your right foot gains ten pounds. :)

With either 3.55 or 3.73 gears the car will pull stronger at lower speeds, but the top end will come in sooner as well. You'll need to get some quality wider tires on the rear wheels to maintain traction. I would not run a tire "shorter" than stock.

Overall you end up spinning the motor more to go the distances you used to (i.e. more wear), but IMHO the performance difference is well worth it. Once you are in traffic & get that first opportunity to punch it from 50 to 70 you will know you made the right choice! Good basic engine maintenance will negate any additional wear.....

Cost?

You can expect to pay anywhere from $600 to $800 for a high quality installation job. The approximate price break down looks something like this:
  • $180 for gears
  • $105 for an install kit
  • $100 for gear oil and friction modifier
  • $200 or so for install and
  • $220 SCT or other tune to for gearing and shift points
The speedometer will be thrown off when you change your gears. A speedometer gear swap is also required to compensate for the gear change. Use this calculator to help you determine which gear you need. See this thread for directions on how to swap it.

The tolerances in the differential must be kept exact, so you should definitely have it done professionally.
 
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