When tires revolve at high speeds, the friction between the tire and the pavement creates heat which expands inside the tire. When the autobahn in Germany was developed in the 1930s, with triple-digit speeds common, tires were not designed to handle the duress. Many fatal accidents occurred due to tire blowouts. Tire manufacturers began to construct tires for vehicles that could travel at higher speeds and handle the stress. These became known as speed ratings. Today, every vehicle on the road worldwide has tires with specific speed ratings.
Speed ratings are located on the sidewall of every tire designed today. The last digit or digits in its tire size code reveals the speed rating for the specific tire. But if the vehicle is used, how would you know if the tires have the correct rating? The best way to learn your correct tire size, load capacity and speed rating is to locate the tire information placard, usually located on the driver's side door jamb.
Most passenger vehicles and light trucks use a letter H for the speed rating on the tires. Referring to a speed rating chart, this tire is speed rated for 130 mph. Speed ratings mean the tire was developed in sidewall and tread strength and then tested at that level of speed, but not necessarily beyond. Speed ratings will always be higher than the potential speed of the vehicle--unmodified--for safety reasons.
When cars are manufactured, they are tested with the size, load and speed-rated tire eventually assigned to the car. Tires with different speed and load ratings handle differently. Maneuvering corners and general handling of the vehicle can be compromised by downgrading in speed or load ratings the tires assigned to the vehicle.
Vehicles that have higher speed ratings have different performance criteria. Many import manufacturers have tires with higher speed ratings assigned to their vehicles for performance reasons.
As speed ratings increase for performance tires, tread life decreases. This is because the tires used for racing or speed are constructed with less tread--or no tread for racing tires--with the goal of speed. So while that BMW you just bought has a W-rated tire (168 mph), why can't you step down to an H-rated tire? You can, but there are factors to consider. W-rated tires are more expensive and last much less longer than H-rated tires of the same size. A W-rated tire is not usually designed with the family car in mind. If you bought the BMW for show and do not expect to race it, stepping down in speed rating can in rating can save you money. However, reputable tire dealers and installers may not be willing to do this for you for liability reasons.
Passenger cars are designed for passengers and performance cars are designed for performance, so crossing paths rarely occurs. However, applying winter tires to a performance vehicle is an acceptable downgrade because winter tires have an average speed rating for adverse weather.
