I'm not 100% sure how to read the middle number when u have numbers such as 215/40/17 I know the first number is how wide the tire is and the last is how big the rim is but whats the middle? I think it has something to do with the side wall?

 

The middle # is the ratio of sidewall to width in percent. So, for your 215/40 tire the tread width is 215mm, and the sidewall height is 40 percent of that.

 

thanks