Tire speed ratings were originally developed in Europe as a way to assist high performance vehicle owners when choosing replacement tires to match the speed capabilities of their vehicles. Tire speed ratings should never be associated with the ability of the vehicle to handle the speed for which the tire is rated.
Today's tires are marked with letters to indicate their speed rating. In the above example, H is the speed rating. Speed ratings are based on laboratory tests which relate to performance on the road. Tires may be marked with one of these speed symbols, M, N, P, Q, R, S, T, U, H, V, W and Y to identify the particular tire's speed rating. Additionally, the letter Z may appear in the size designation (see chart below).
When a replacement of your tires is required, consult the vehicle manual or tire placard for proper size and speed rating (if required).
If the vehicle manual placard specifies speed-rated tires, the replacement tires must have the same or higher speed rating to maintain vehicle speed capability.
If tires with different speed ratings are mounted on the same vehicle, the tire or tires with the lowest rating will limit the tire-related vehicle speed.
Tire speed ratings do not imply that vehicles can be safely driven at the maximum speed for which the tire is rated, particularly under adverse road and weather conditions, or if the vehicle has unusual characteristics. Never operate a vehicle in an unsafe or unlawful manner.
*For tires having a maximum speed capability above 149 mph (240 km/h), a "ZR" may appear in the size designation. For tires having a maximum speed capability above 186 mph (300 km/h), a "ZR" must appear in the size designation. Consult the tire manufacturer for maximum speed when there is no Service Description. Example: P275/40R17 93W at 168 mph (270km/h) or P275/40ZR17 at above 149 mph (240 km/h). Consult tire manufacturer.
**A "ZR" may appear in the tire size designation.
NOTE: For "V", "W", or "Y", and tires with a "ZR" rating, a vehicle load adjustment (for speed) is required; consult tire manufacturer.
Source: Rubber Manufacturers Association
Remember, if you are checking the tire pressure, it's important to use an accurate tire gauge.