An alpha-numeric symbol you'll find on your tire's sidewall that tells you the maximum sustained speed the tire is capable of safely handling. An H-rated tire, for example, is built to be safe for continuous operation at speeds up to 130 mph. Most current model year family-type cars have S (112 mph) or T (118 mph) speed ratings. High performance cars often have tires with a V (149 mph) or ZR (in excess of 149 mph) speed rating. A few ultra-performance cars have W (168 mph) and even Y (186 mph) speed-rated tires.