Tires on wheels?
 

just a quick noob question.. would 255 and 245 fit on 17x 7.5 wheels ?

I'd say you could mount both of them on those wheels, but it's certainly nowhere near ideal. Stock class auto-xers do similar size differences all the time. That said, you'd be much, much better off going with an appropriately sized tire (215 or 225). More response, more grip, and better feel.

+1. Why would OP want to mount tires that wide to begin with. Wider is not always better.

thanks i have 235s on now.. feels great. just not as flush as i would like

Depends on application, some also like the look. For instance im a straight line kinda guy, and ill be doing a high hp build. I'm probably gonna be rocking 255/40/18's and 285/40/18's on 18x9's and 18x10's respectively. As a low boost street setup.