Originally Posted by Camrann
235 is the tread width and the 40 is the side wall height if im not mistaking
The 40 indicates that this tire size's sidewall height (from rim to tread) is 40% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall.
Taken from tirerack and changed the 50 to a 40 to be pertinent to this thread. Simply put, it's a ratio not an exact measurement like the width.