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You & Your Car

Understanding tire speed ratings Add to ...

I’d like to know how critical it is to follow the speed rating on a tire when replacing it. What is the impact if a car’s original specs are for an H rated tire and it is replaced with a T rated tire? Any impact on mechanicals/drivetrain/suspension? If it’s critical, why do dealerships recommend getting S rated winter tires? – Silver

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While I wouldn’t call it critical to replace original equipment tires with ones carrying the same speed rating, I would suggest there are reasons for doing so.

It might help if I explained “speed ratings.” The rating does not tell you how fast the tire can go, rather how long it can be expected to last at a sustained speed.

The biggest safety issue with respect to tires is heat. An underinflated tire will generate more heat; a worn tire will generate more heat because there is less tread to absorb the heat. A brand-new tire will generate heat as the sidewall flexes not only when the when the car goes up and down over bumps and during cornering but principally as it rotates.

A tire, depending on size, will rotate in the 750-900 times per minute range at highway speeds. With each rotation, it goes from compressed at the bottom where it contacts the road to expanded at the top where centrifugal force pushes it out. This constant movement as the sidewall flexes generates heat. Too much heat and the sidewall will blow out.

The more weight or pressure there is on a tire, the greater the forces acting on the sidewall and the amount of heat generated. Those big pieces of tire you see along the highway are a result of heat. Most often they are treads thrown from retreaded tires when heat built up to the point that it caused the adhesive used to bond the tread to the tire to give up

Heat is tire enemy No. 1. With this in mind, there is a rating system that tells us the maximum speed at which a tire can run for two hours without failure. The chief reason for these ratings is the unrestricted speeds of the German autobahn.

The ratings run from M for 130 km/h to Y for 300 km/h. In North America, where the maximum legal limit is 75 mph or 122 km/h in some U.S. states, virtually every passenger car tire sold exceeds that limit.

The S rating you mention is 180 km/h, T 190 km/h and H 210 km/h. So on the surface, there is no need to buy a tire with a higher rating. But it is not that simple. Vehicle loads and inflation pressure directly affect how much a tire will flex and the amount of heat generated, so there is a need for safety margin. And, during the development process, suspension engineers designate a tire size and speed rating – often higher for cars that may see use on the autobahn.

That rating also takes into account the size or depth of the sidewall, the “profile” of the tire. A lower profile means a stiffer sidewall and less flex and heat. Tires rated for extremely high speeds, without exception, have a very short sidewall. Tires with a low speed rating tend to have very deep sidewalls that will flex a lot more and create more heat. And tires with a higher speed rating have a stiffer sidewall and are more responsive to steering input and better under severe braking.

The bottom line – you should stick within a pretty narrow range of that original speed rating – incorporated into the design of the vehicle.

The ratings

  • M 130 km/h
  • N 140 km/h
  • P 150 km/h
  • Q 160 km/h
  • R 170 km/h
  • S 180 km/h
  • T 190 km/h
  • U 200 km/h
  • H 210 km/h
  • V 240 km/h
  • Z 240 km/h plus
  • W 270 km/h
  • Y 300 km/h

Dead battery

I left the lights on overnight in the car and killed the battery. After a jump start, how long should I run the car to recharge the battery? – Joe

Most of us have done that once or twice. You should run the battery at least 20 minutes and try to do it on the road, not when the car is sitting still and idling. The faster engine speed will allow the alternator to turn more quickly and supply more current to the battery.

Revving the engine while the car is still is a waste of fuel and can cause excessive heat as no air is moving through the cooling system.

globedrive@globeandmail.com

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