Someone had this link on the forum before www.miata.net/garage/tirecalc.htmlIt shows thecomparison between sizes in illustration of height & width and any changes to the speedo the size may cause.The factory size for my 07 R/T is 225/60/18.You figure the factory tires for an SRT are 245/45/20. Using SRT sized tires and 20 inch wheels, my speedowill read 60 mph but my actual speed will be 60.1 mph the width & height changes are negligible. I suggest checking out that site prior to any tire or wheel change.

 

Yea Im running Toyos 255\45\20 we figured the speedo would be off justa a hair .I tried to get the sidewall height as close to original as possible. Also so we wouldnt loose as much ride quality by going to the 20's