changing code for wheels?

Hey, i got a 02 alero 2 dr GX i belive, and i just put 17's with toyo rubber, and my GPS is telling me my speed is different than my speedometer is telling me, and with the factory wheels it was almost identical. So i am guessing that is always screwing up my KM reading as well. I know that means i got to upgrade the computer and change the wheel size, but does anyone know how? i dont even know where to start looking for that. thanks guys!

[Kilroy]

You could always go to a lower profile tire so that it ends up being the same size as before.

[meadus101]

^ Not to mention looking badass.

[AGT]

Yep. If you want your speed and mileage to stay accurate then you will need to make sure that you keep the same circumference