Sidewall Height or Aspect Ratio
The aspect ratio or profile of the tire is calculated as the sidewall height expressed as a percentage of the tire width. So a tire with an aspect ratio of 80 is a tyre whose height is equal to 80% of its width. Modern tires have lower aspect ratios. The expression "sidewall height" is more accurately defined as the section height – the radial distance between where the bead of the tire sits on the wheel rim to the outer most point at the centerline of the tread.
This is the width across the tread measured in mm - the width of the inflated tire section, excluding any lettering or decoration on the sidewall.
A tire's overall diameter is the outer diameter of the tire measured in the center of the tread. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours.
The distance around the outside of a tire is called the tires circumference.
Revolutions per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.