Right now I have the stock tires P215/55R18 94T, and they need to be replaced. Ive got a spare set of tires P235/75R15/ 108S that I would like to put on the car, but I'm unsure if these will cause any problems since they're larger tires.

 

http://www.1010tires.com/tiresizecalculator.asp

I'd say no 3% is about the limit those are almost 6%