what do you think the biggest tire size i can fit on a 17x7.5 inch rim? trying to put wider tires on the rear next spring.

 

Depends on how much bulge you want to deal with. 255 is probably as far as I would take it. Anymore then that and your getting diminishing returns anyway, due to less and less tire contact patch. You would have to run pretty low pressure to get the full width of the tire back on the ground and your going to end up with a bulging, soft, shitty handling tire. Be ok for strait line/drag racing though />

 

235 is the biggest I'd put on it (225 would be a vertical side wall for a 7.5inch width). Anymore and you'll lose too much responsiveness.