If you want to go up in rim size you need to go down in your sidewall size to keep the same rolling diameter. I found a website that will help you figure out what tire size you will need to keep your rolling diameter the same or close. All you have to do is put in your stock rim and tire size and under "Compare Tire Sizes" put in what you think is the correct rim and tire size. Then hit "compare tire sizes" and see if your correct if your off just play with the sidewall size till the Overall Diameter is the same or as close as you can get it. The calculator will even tell you how much your speedo will be off and alot of other great info.

Rolling diameter calculator

Good Stuff! It's darn near the best all around tire calculator I've found.

good find Jedi3!!!

Been using that for years, works like a charm. Much better than trying to make some shops word for it.

Thanks for the link I used this when buying tires for the truck this past weekend.