whats everybody doing when they change tire size about the speedo going back to the dealer ifso whats the cost does anyone know how much they were off say 17' to 18' 18'to 22' can't be much from 17' to 18' now 18' to 22' that may get you a ticket

 

SuperChips or other like tuner will allow you to modify the tire size within the cars computer..

 

Only takes 5 to 7 mph for a ticket

 

we switched to 20"s but when we did I calculated my diameter and kept them almost the same

 

Baldy,
I just answered my question about your wheel sizeon another thread by reading this thread.

I have been busy figuring out what works and does not work. I dont remember where I came across the link(this forum or anotehr)but it rocks....it's a JAVA based tire size calculator.

For myself going from 18" wheels to 20" I chose a tire size of 245/45R-20 which is 0.2% slow meaning if my speedometer is reading 60mph, I am really going 60.1mph. I can live with that .1mph error

Looking at going with Goodyear Eagle-RSA 245/45ZR-20 speed rating upto 186mph.

Here is the link:
http://www.miata.net/garage/tirecalc.html

 

Tire dealers also have nice tools for getting the information you after. You can also use this(non-JAVA) calculator: http://www.discounttire.com/dtcs/infoTireMath.dos