LRP=Limerock Park Lakeville CN
NHIS=New Hampshire International Speedway Louden NH

I have a 97e36m3 that I am currently running in the ITE class in the NER region of the SCCA. The car has been an ongoing effort, and this winters project will encompass a new diff for the car. Being in the planning stages, I am looking for folks who may have already done this and the recommendations they might have for ratios / slip . I have already had suggestions for a 4.10 for New Hampshire and Limerock and a 3.73 for the Glen by a major diff supplier. However, the more I search, the more varied recommendations I receive. Other candidates have been a 3.91 for New Hampshire and Limerock and a 3.64 for the Glen.

The car currently is putting out about ~265 bhp at the wheel, but this too
is an ongoing effort to raise (too many killer Porsches in my run group!)

Any suggestions are welcome. Thanks in advance.

What size tire are you running in the rear and how much does your car weigh?

I use a 3.91 at lime rock and NHIS, and a 3.73 everywhere else. It taps at the glen for a brake marker or 2. I'm running 280/640/18s, basically 275/ 30/ 18s. If your running more of an I prepared set up, those should work for you. I could go as low as 4.27 at lime rock...

Thanks for the reply. The cars weight is
2753, and I am running 235/40/zr17 RA-1's.

I use a 3.91 at lime rock and NHIS, and a 3.73 everywhere else. It taps at the glen for a brake marker or 2. I'm running 280/640/18s, basically 275/ 30/ 18s. If your running more of an I prepared set up, those should work for you. I could go as low as 4.27 at lime rock...

You will want to run 280/640/18 FRONT and 280/680/18 rear on your new car. The rear fenders are designed around that tire size and the higher profile rear will help!

To answer the OP's question, I have another question? Will you be running two different diffs, or one compromise diff?