but i ask cause a 315 on a 20 is fn huge! way too much tire for a 20" wheel on a 98!
I don't think you are fully understanding
315 has nothing at all to do with tire size, that is the tread width
A 315 wide tire on a 17" will be the same 315 wide on a 20", it's the sidewall that changes. If you use a correct ratio you can still end up with around a 26" overall diamter which will fit fine.