What the Heck Do Those Tire Numbers Mean?

The numbers in the tire size refer to its width, aspect ratio, and wheel diameter. "275/35R19" means the tire is 275 mm wide, has an aspect ratio (that is, the width divided by the height) of 35%, and fits on a 19" wheel.

You can figure out the height of the sidewall by multiplying the section width (275) times the aspect ratio (35%). The sidewall height of our example tire would be 275 * .35 = 96 mm, or about 3.8".

"R" means radial tire. They're all radials nowadays. Speed ratings are usually inserted before the "R." The most common speed ratings are HR (good for 130mph), VR (149 mph), and ZR (more than 149 mph).