Quote Originally Posted by parakitoJDM View Post
u should compute by over-all WHeel diameter. meaning wheel+tire.
i think the best way to compute them is with a GPS. probably Top Gear Philippines or "C Magazine" can help on this....
Agree with this. It should be the overall wheel diameter. Ideally, it doesn't matter even if 15" or 17" mags as long as the right profile of tire will be used to maintained the overall wheel diameter.

The problem with the stock 15s I think is the high profile of the tire. After a while, the rubber sidewall sags even if you maintain the tire pressure. So in effect, you will have a shorter wheel radius, a little bit higher speedo readings than actual and a lower ground clearance

The difference with using a 17s mags is that the rubber is less and therefore the sag is less also. Metal don't compress as oppose to rubber. And therefore, you will have a more accurate speedo reading using 17s. Mine even have an oversize tire, mas higher ground clearance at mas matipid pa sa fuel