TIRE SPEED RATING AND WHY IT MATTERS
The tire speed rating is the maximum speed tires can safely carry a load (the original weight of your vehicle plus whatever's in it) for a sustained amount of time in ideal conditions. The ratings is molded on the tire sidewall, signified by a letter or two, usually after the load index number. Together, the load index and speed rating from the services description.
Each letter in the speed rating represents a maximum speed based on a standard chart.
THE MAIN THINGS TO KNOW:
THE MAIN THINGS TO KNOW:
- Generally, the higher in the alphabet a tire is rated, the better it will manage heat and faster speeds. There's an exception for the H rating; read on for why.
- Your actual speed capacity may be less than a tire's rating. The rating indicates a new tire's performance in tightly controlled lab settings, not the open road. Tire condition, inflation level, extra cargo, road surfaces and weather are everyday limits that play into a tire's maximum safe speed.
- If you have tires with different speed ratings, the limit of the lowest rated tire is the fastest you can drive and stay within your tire's capability.
- The most common ratings are S and T (sedans, minivans, light trucks); H (some passenger cars, sports cars, coupes, some light trucks); N, P, Q and R (light trucks); and V, W and Y (high-performance cars). Most winter tires have Q, S or T speed ratings.
HOW ARE SPEED RATINGS DETERMINED?
Tire manufacturers determine a tire's capacity for heat and speed using a testing machine. Usually testing is done to meet ECE (Economic Commission for Europe) standards, so the scale is based on kilometers per hour (km/h). A more rigorous test is sometimes done to meet SAE (Society of Automotive Engineers) standards.
- For the ECE test, the tire is properly inflated and mounted on a wheel attached to a testing machine in a room that's heated to 77 degrees F.
- The tire is pressed against a metal drum with enough pressure to simulate a realistic load.
- Starting at a speed 40 km/h lower than the proposed rating level, the tire is spun for 10-minute increments at higher and higher speeds, until it reaches the target speed.
- The tire spins for 10 minutes at the target speed.
- It's then removed from the machine and inspected for any failures, like separation of tread components. If it's intact, it passes the speed rating.
- Sometimes an SUS (step-up speed) test will be added after the tire performs at the target speed rating. Speed is increased until the tire fails.
- If the tire is being tested at the SAE standard, its required to run for an hour at target speed in a room heated to 100 degrees.
HOW MUCH DOES IT MATTER FOR YOUR DRIVING & TREAD LIFE?
Tires with higher speed ratings offer handling benefits that thrill some drivers, but there are tradeoffs. Since they're usually made with softer rubber compounds and stiffer construction they offer better cornering, stopping power and steering response. But expect a little less ride comfort, lower performance in cold conditions and shorter tread life. Consumer Reports found that some H- and V-rated tires didn't last as long as those rated for lower speeds, wearing out closer to 50,000 miles than 60,000 miles.
WHAT RATING DO YOU NEED?
Even in states where rural speed limits are 75 mph, most drivers will stay well below the speed limitations of H-rated tires. Commuters and family car drivers will likely be quite satisfied with S or T tires.
If you're a spirited driver with a high-performance car, you may be happier with V, W or Y tires. Light truck drivers will be looking for symbols N, P, Q, R, S, T or H.
If you're replacing tires and think you may want a lower- or higher-speed-rated tire, it's best not to mix and match. When mounting differently rated tires, techs prefer to put the lower-speed-rated ones on the front to prevent oversteering. This can conflict with the best practice of putting the tires with the most tread on the rear, which is important for wet traction.
Get tires with the same speed rating. If you don't, remember that the maximum mph is limited to the tire with the lowest speed rating.
Tire speed rating is not like a speed limit posted on highway signs. It's based on lab simulations and doesn't account for real-life factors that determine true tire capability: Are the tires fully inflated? Is your vehicle properly aligned? How hot is the road? Have you ever had a flat repaired? What's the age and state of the tread?
Don't use the rating as a guideline for the top speed you can drive. If you're carrying a heavy load, have a tire that's been patched after a puncture, or low on air, your tire's speed capacity will be much reduced. For safe driving, keep your speed at the posted speed limit — or below, when road conditions or the weather aren't ideal.
Important Notice: The information provided above is derived from sources deemed reasonably reliable. The operation of your vehicle, or the replacement of your vehicle's equipment, may be different than for a typical vehicle. Please consult your owner's manual for specific warnings, notices, and other advice relative to the subjects addressed.