i thought about the 3.55 but ultimately decided against it. Since our power goals are 300 hp I went with 3.27
3.27 will get you 121 with three gears with 275/35/18 at 5200 rpm. Our plan is to not use OD at all. 3.55 only gets you 111 at 5200 which may force you into OD at some tracks Id say 5200 is a decent limit for fuel economy and reliablity. IF you need a little more mph then a bump of the rev limit is all you need. 5500 is 128 which is probably enough for a 300 hp 3100 lb car.( ours is probably close to that since 1996 is fairly light to begin with ) Worst case scenario you can just use OD and be fine.
If you plan on a 5 speed then a 3.55 or 3.73 may be beneficial.
I snagged a used Auburn road race LSD and it works fantastic. Its a spring type not a clutch based unit. Lucklily it was only $100 since its a 28 spline but will handle 300 hp all day long.
We actually did the same calculations. We rarely hit 111 (at least so far) but feel like the trade off for us is better power out of the turns. We also only run in the first 3 gears.
We’re running Thunderhill at the end of May, which is pretty much the fastest track we run. We’re still on 3.27, so this will give us a better idea. Either was a LSD is definitely in the future.
Thanks for the tip on the Auburn. They make great products and I would not hesitate to buy something made by them.