Learning to read a tire's sidewall information will allow you to use the tire for its intended application. This also means that you can safely make use of the tire on an appropriate vehicle.
Any letter in the tire sidewall spec besides the "R" identifies the tire's "Speed Rating." The Speed Rating is a chart of letters that designate at what speeds the tires can be safely used at. For example, a "V" stands for safe speeds up to 149 MPH, while the "Z" in the chart means a tire is designed to be safely ridden at speeds in excess of 149 MPH.
The "R" letter in the tire spec stands for "Radial," which designates the tire as a radial tire. Virtually all modern tires are of a radial design, save for tires designed for race track applications and other specialty purposes.
Virtually all modern tires are rated "VR," so this is the most oft-seen letter string in tire sidewall specs. You might see other letters combined with the "R," which means the tire is rated at a different speed. This won't usually matter, as most people don't ever drive over 87 MPH. If you have a set of tires that you want to find the speed rating for, see the link in the References section for a table showing the various speed rating designations.
