I recently put on new 18's with 235/40/18 and I'm wondering if it would affect my speedometer and if so how much , would I be going faster that the display or slower ?

 

If your original size was 225/50/17, the speedometer would read roughly 1.8% slow, because the overall diameter of your 18's is slightly smaller. The speedo would say 60mph but your actual speed would be 58.9mph. Go to miata.net's tire calculator and you can play with the numbers yourself. It is a very useful tool.

 

that would mean that his speedo is reading about 1.8% too fast.

 

Did I say slow? I meant fast. I hope he looked at the mph instead.

 

Haha, if you got smaller wheels you could use that as an excuse of why you were speeding. Of course you would have to have 2inch rims to be going 35 over.

 

LMAO