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Tire speed ratings are determined in laboratory testing. These ratings are also evaluated by consumer groups and car enthusiasts-people with a vested interest in knowing the exact limitations of their equipment. One fact often overlooked about tire speed ratings; these ratings apply to undamaged tires only. There is no way to determine the tire speed ratings on a damaged tire as each kind of damage produces different results. It's best not to attempt any extended or "at speed" driving on damaged equipment. When your tires are compromised, it creates conditions suitable for potential additional damage. Pushing your tires too hard in such a condition makes for unsafe driving. This caveat also applies to altered or modified tires. No data is officially available on the tire speed rating based on such modifications.
Tire speed ratings give no real indication of how a given vehicle might perform at these speeds. Remember that your engine, suspension and fuel consumption play a major part in how the vehicle operates at speed. In some cases, it is conceivable that you could damage your engine before reaching the maximum rated speed for your street-legal machine. Race cars are built differently and made to last; consumer vehicles react differently to this kind of abuse depending on the duration and frequency of excessive speeds. Your tires may be able to handle much higher speeds than the rest of your vehicle is able to endure, and there are no guarantees as to how your vehicle may be affected at those prolonged higher speeds.
Tire speed ratings are assigned to tires based on a series of test conditions. Some people see a tire speed rating and assume they can drive their vehicles as fast as the rating allows, anytime. Drivers often miss the fact that tire speed ratings are based on near optimal conditions-perfect temperatures, safe conditions, etc. Drivers in Texas won't get the same performance from a tire in the summer heat as a driver in Canada in the dead of winter. Remember that speed means an increase in heat as the tire rotates on the pavement. Add summer heat, road debris, and other variables to the equation and you get a big caution flag from the laws of physics--don't try to approach the maximum speed rating of your tires under these conditions, you may get a lesson about heat physics the hard way.
A tire's speed rating depends on how long it can safely perform at a given speed. As a tire is driven faster and faster, it generates heat, which can deform the tire structure. High-speed tires are designed to resist these deformities at the speeds they are rated for. Resisting deformities means controlling heat buildup. The only way to do that is through innovative belt design, which all tires have as appropriate to the speeds they are built to handle. Tire speed ratings are designed to let the driver know what speeds are safe--not as a suggested guideline that can be exceeded with no worries. Safety is the major concern with such rating systems, exceed the recommended tolerances at your own risk. Tire companies assume no liability for accidents caused by exceeding the rated speed of a given tire.
Tire speed ratings do not apply to repaired or modified tires. The original rating is done to an intact tire and does not reflect the performance of a compromised tire. Opinions vary, but you can probably only safely get 75 to 80mph out of a patched tire. Those who routinely push the speed limit should replace punctured or damaged tires, not repair them. Don't forget that the speed rating of your damaged tire goes down further with heat, low pressure, and other factors. Tire speed ratings are assigned in optimal conditions, there is no official data as to how tires perform when the structure is damaged, patched, and put back into service.
If you are purchasing a used car, check the speed rating on the tires to make sure you know what you are getting into. If your tires are 'low-end' models on the tire speed rating chart, you'll want to know just how much of a lead-foot you can be and still stay within safety margins. This is important to careful drivers as well as those with a need for speed; if you find yourself in a situation where you need rapid acceleration or sustained acceleration, you'll want to know what the safety margin is. Don't forget that hot pavement in summer weather can alter that safety margin. If you need to seriously pick up speed to avoid a rear end collision, how fast can you safely go? Your tires will tell you. The sustained acceleration scenario is possible in a number of situations-rushing a pregnant or injured person to the hospital, for example, or trying to make speed to avoid a rear-end collision with a truck or trailer.
High tire speed ratings are well and good, but the average driver should question whether the trade off of comfort and noise level is worth the additional speed potential. With increased performance comes decreased comfort. This is acceptable in the world of racing, but the average driver and even many custom car buffs feel comfort is just as important. High noise factors in performance tires can send some drivers fleeing for the safety of a more moderate-performance model. You may not be able to reach "hair on fire" speeds with a lower tire speed rating, but you will arrive at your destination in comfort and ease. Check out the consumer report and review websites for a good sampling of experiences with different tire speed ratings.
Drivers who get stuck in snow, mud, and sand will spin the vehicle's tires in order to try and make enough traction to get out. In situations like these, tire speed ratings do not reflect an accurate safety margin. Never spin your tires faster than 35mph. Spinning tires can break apart and explode above those speeds, regardless of what your tire speed rating says. The cause? Centrifugal force. It can tear apart a tire at speeds higher than 35mph, often in an explosive manner. Pay close attention to the speedometer when trying to get out of a snow or slush rut, and make sure no one stands behind the vehicle as you attempt to get out. Tire speed ratings are not calculated under such conditions, and therefore don't apply. Use the “under 35mph” rule for maximum safety.
Tire speed ratings are simply the maximum amount of miles per hour a tire is rated to handle. These ratings are helpful in European countries where there are no speed limits in some areas. In these sections of road, there is no such thing as speeding, and knowing the limitations of your equipment is very important. Many wrecks along Germany's Autobahn are due to a lack of understanding of a vehicle's limits.
In America, these ratings are still listed on the sidewall of all tires, but tire manufacturers do not advise people to exceed the legal speed limit, no matter what the tire's speed rating may be. The ratings are listed as a safety guide, not an invitation to break local speed limits!