wheels
 

What are the best wheels/sizes to run 245/40/17 front and 275/40/17 rear on a '91 C2?

Turbo or N/B?

I don't know if that size will work up front, that's very wide for any 964 Carrera, (turbo's and N/B's are the same up front). You would need 17x8's but there some real potential for rubbing issues, or you'll need some funky offsets.

If it's a N/B -
In the rear 275's on 17x10's should work, but you may want to go to a 30 or a 35 aspect tire. Again offsets are key so you don't have rubbing near oil lines on the passenger inside. Probably need offsets around 47mm +/-.

On my N/B - I am running OE C2 turbo wheels - 17x7's et 55 up front with 225's right now and will be running 265's on 17x9's et 55mm in the rear shortly, they should fit perfectly.