Rear Differential Ratios

[Vincenze]

What cars did the GS6-37BZ come in? Would it just be cheaper to do an engine swap?

The M20 is slanted less: 20°.
The M50, M52, M54 are slanted 30°.

The models that the GS6-37BZ came in:
https://www.realoem.com/bmw/enUS/partxref?q=23007532498
https://www.realoem.com/bmw/enUS/partxref?q=23007565188

It's expensive, yes.
But if you want to do an engine swap in the future, you'll face the same problem.

[jeffnhiscars]

3.46/3.64 is your best bet for all around, but they're rare and expensive. The only chance you have at a cheap 3.64/3.46 ratio is out of a Z3 from the junkyard.

3.73 is what I run on all of my cars and I do plenty of highway cruising, works fine for me.

Also re: the 3.91 above, they're also quite rare in the US, as they really only came in manual iXes and a very small number of other cars.

We snagged a 3.46 that comes out of an 01-03 Z3 1.9 w auto trans. Its a torsen and it went in to our eta. it felt just right until we installed the built 731, now its about perfect :)