Hey, i just bought SI rims, and i was wondering if i need to do a speedo recalc. THe SI's are 15", while the old stock steelies are 14". will i need to get one done, and if so, how much does this cost?

 

No. With larger wheels you will need lower profile tires (probably 55 for 14", and 50 for 15") so that the overall diameter of the tire remains the same.

 

a good tire size for the Si rim is 205/50/15 or 195/50/15. Stock is 195/55/15, but you'll be happier with the shorter sidewall

 

The sidewall of a 205/50 tire is only marginally shorter than a 195/55 tire.

The "series" of the tire (50, 55, whatever) is the sidewall height expressed as a percentage of the section with (205, 195, etc). So a 205/50-15 tire is 205 mm wide, and its sidewall is 102.5 mm thick, while a 195/55 tire is 195 mm wide and has a sidewall thickness of 107.25 mm.

That said, I'd still go with the 205/50-15 because it is a bit of an improvement.

This is a good resource for comparing overall diameters of different tire sizes: http://www.miata.net/garage/tirecalc.html