Where can I find info that will help me adjust the speedo to compensate for bigger tires? I had the 17in Pirelli (stock) and now I have 18 SSR with 255/45ZR/18 Goodyear F1s all the way around.

Thanks

 

well you could always do the math. calculate the circumference of the tire and and take a mile and divide it by that.

Circumference is pi x diameter.

 

www.tirerack.com will list revs per mile for each tire.

 

255/45 is a 11.475 cm sidewall which converts to (4.5177 inches x 2) + 18 inch tire is diameter of 27.0354 inches x pi(3.14159265) = 84.9342 inches

1 mile = 63360 inches. / 84.9342 = 745.98 revolutions.

 

Bruce, I tried to do the same with mine. I adjusted it with the SCT Xcal2 (I think I used 745) and it seemed to mess up the speedo. I put it back to stock and it was fine. It's still set to stock with the Goodyears and it's dead on with the mile markers and speed traps.

 

Thank you very much Steve...that was what I needed to hear. The Tire Rack does not list the revs per mile for this tire.

 

didn't trust my math.

 

Plug the two tire sizes in at Miata.net Tire Size Calculator

I get only a 0.5% (742 vs 746) difference... no change necessary.

 

I did 748 revolutions in the parking lot today on the way out of work, and its a small parking lot

 

you can also visit the tire manufacturers website, In this case BFGoodrich publishes its tire data http://www.bfgoodrichtires.com/specs...-a-kdw/22.html (as an example)