How do you do this?

Ex; 275/40/17 would convert to what in diameter and width, and how do you convert it?

 

ive always used
http://www.miata.net/garage/tirecalc.html
hope this helps

 

The 275 is the width of the tire in millimeters, in this case it equals 27.5 centimeters.

The 40 is the aspect ratio, as in the sidewall height of the tire is 40% of the width. Therefore, that tire would have a sidewall height of 110 millimeters or 11 centimeters.

The 17 is the diameter in inches.

1 inch = 2.54 centimeters, 1 centimeter ~ .3937 inch (1/2.54)

Scott