does anyone know if i buy new wheels say 18x8.5 front and 18x9.5 rear will tht effect the speedometer in any way? will it throw it all off?

 

It depends on the tire size.

Check out the "Official Wheel and Tire Fitment"
https://www.clublexus.com/forums/sho...d.php?t=158592

 

If it does, the only way to re-calibrate is to use GPS.

 

what about these tires woud they effect the speedo in any way?
Front: 245/40-18
Rear: 265/35-18

 

If you shrink the diameter tire size (height) of the tire compared to the stock tires the speedo will read just a hair to fast...If you increase the size over stock the speedo will read a hair slow.....Because the if the tire is shorter you will get more revolutions (turnover) then as compared with a larger stock tire which the speedo is exactly calibrated too... Comprende Amigo? Any trie change with 18 inch rims should not make a noticeable difference....Unless you get crazy low profile tires on the 18's.. I have 18's with 275/35/zr18 tirs which are low profile and I didn't notice any change with the speedo....that shrunk down my wheel height...

 

Those tire and wheel combinations will increase the total diameter of your tire/wheel by about 1.5 inches. Like the guy above said, that will cause your speedo to read just a hair slow. Because of the increase in circumference, the car will travel farther per tire revolution than on stock wheels/tires.

 

How did you come up with 1.5" to large?

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

A) The front tires dont matter as far as speed goes. B) 265/35/18s will be .2" too small making your speedo 1.7% off (reading 60mph when you are actually traveling 59mph).....or so says my very well trusted miata tire size calculator.

 

I don't know about the miata tire size caluclator but...265/35/18's are (265 * .35 ) to get the side wall thickness. That number times two(you have two sidewalls in the diameter)...then converted to inches. Add that to 18 to get the new diameter. I used 225/45/16 for stock tires. I believe that it gives you 1.5" greater in diameter. Could be wrong though?