As a car owner, you might know to never mix and match tires with different speed ratings — unless you want to seriously damage your car. But what do speed ratings mean? How can you know what your car’s speed rating is? And how do you know if your tires match the speed rating of your car?
Speed Ratings: More Than Just Speed
Tires do three things — carry, transmit and steer. Speed ratings matter not only in relation to speed, but it also affects the comfort of the ride, the car’s cornering ability and overall wear. Typically, the higher the tire’s speed rating, the better it is at stopping and gripping the road. It will tend to have a lower tread life though. You can increase the speed rating of your tires for improved vehicle performance; however, you can never decrease the tire rating without reducing the vehicle top speed accordingly to match the lower speed rating that has been chosen.
Origin of Speed Ratings
Tire speed ratings originated in Germany’s famous Autobahn. Speed ratings can range from the very lowest (A) to the highest (Y). Keep in mind the speed chart is not listed entirely in alphabetical order For example, the rating of H is listed between U and V due to the perception that H has stood for “high performance.” As cars continue to get faster, tire speed ratings must continue to evolve to match their speeds. At one time, Z was the highest rated speed of over 149 mph. More recently, W and Y were added to indicate the higher speeds reached by more recently developed exotic sports cars.
Dangers of Mixing Speed Ratings
If tires having different speed ratings must be used on a vehicle, the lower speed-rated tires should be placed on the front axle — whether the vehicle is front wheel or rear wheel drive. As a result, a potentially dangerous oversteer condition is prevented. Keep in mind the vehicle’s handling could also be greatly affected. The car’s speed capacity will now be limited to that of the lowest speed-rated tire on the vehicle.
For peak vehicle performance, the same kind and size of tire should be used on all four of the wheel positions. For instance, if Goodyear tires are already on the vehicle, choose the same brand for any replacement ones.
If you see 235/55R17 99H on a tire, the load index would be 99. This is the tire size’s assigned numerical value, and it is used to compare tire relative load carrying capacities. The higher the load index number, the greater the tire’s load carrying capacity.
- 99 = 1,709 pounds
- 98 = 1,653 pounds
- 97 = 1,609 pounds
Typically, the load indexes of passenger car tires and the tires of light trucks range from 70 up to 130.
Make tire ratings part of you basic auto knowledge for peace of mind as a car owner and consumer. If you have more specific questions about your vehicle, ask a tire expert about the best course of action for your car.