How do you find the diameter of a wheel?

[REDSXYGT]

What's that formula to find the diameter of the wheel? I'm trying to figure out these diameters: 275/40/17 or 275/40/18

 

[Wi-Stanger]

the 17 is 1 inch shorter than the 18, the number 40 in that is the sidewall height. As for actual diameter I'm not sure.

 

[00s281sc368]

275 is the tire width on the road, in millimeters, divide by 25.4 to get in inches
40 is the sidewall height, in percentage of the width, so it means 40% of 275mm
18 is the diameter in inches...units are kind of mixed up ...

275 = millimeter
40 = %
18 = inches

So the wheel diameter in inches would be :

275/40/17: 17 + (275x0.40/25.4 x2) = 25.66 inches
275/40/18: 18 + (275x0.40/25.4 x2) = 26.66 inches

 

[nickthegenius]

Here is a handy calculator. Yeah, its on a Miata site, but it works

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