booo, go 37" mud boggers!
You have to keep the same ratio. Other things to consider (other then the car looking retarded with normal profile tires on 15-17's) is when you go to a bigger tire that isn't the same (realitive) diameter is your speedometer will read alot slower then what you are actually going. Same thing if you get a lower profile tire ratio then your OEM wheels, your speedometer will read faster then what your going.