What are the stock rays bud ?, and how comes then some companies have a smaller profile at the rear
Well there is a bit of room to play with, but as you'll have seen on many threads on here, you need to keep it pretty accurate.
I beleive Rays are 225/45/18 and 245/45/18. Thats just the Rays though, the other std wheels may have different RR, just they keep the ratio of front to back the same. If you do a search I posted pretty comprehensive numbers before of the ratio you need to keep. If all else fails though, aim for what you have now in terms of RR