About to get 18" wheels and need advice about tire diameter and speedometer reading. 215/40 245/35 tires are about the same diameter as the stock tires, but rare and hard to find in matching set. So I will probably go with 225/40 255/35 which are much more available, but are about 1/10 to 1/8 of an inch larger diameter. How much will this change the speedometer reading at say 70 mph? Is there a formula for figuring the exact difference?

William

 

http://www.s2000fans.com/faqs/faq_wheels.php

 

Try this page it calculates the stock tire to the new tire.

http://www.miata.net/garage/tirecalc.html

 

Thanks Taimur,

Great little calculator. So the answer is for 225/40 255/35 is .681% to slow or 70 mph reading = 70.5 mph. 215/40 245/35 is .428% to fast or 70 mph reading = 69.6 mph. Not much difference either way.

William

 

or 285/30-18