I have done some searching and still can't figure this out. I have a 2K2 SE with the stock 17" wheels. I want to get 18" but how will much will this slow down my car (1/4 mile, etc.) Assuming that the 18's are light to medium weight.

 

Not sure on the calculations, but it would also depend on the diameter of the tire on them. If a very short sidewall (35 or so) you might actually keep the overall tire/wheel diameter the same so no effect on speed.

 

18's will probably slow you down a little bit. To find the optimal tire for your car check out this site for tire calculations. http://www.miata.net/garage/tirecalc.html

 

If its light 18"s, it won't slow down your car at all. I just got a new set of 18" Dronell and my car is accelerating much easier than with my Borbet 17"s. Just make sure that the wheels are forged. Mine are 2 pcs. forged