Tire speed ratings - Subaru Forester Owners Forum
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post #1 of 4 (permalink) Old 03-31-2006, 12:27 PM
jmacmaster
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Tire speed ratings

I've seen some disagreement on this forum over whether one MUST or SHOULD always run tires with the speed rating recommended by the manufacturer or the speed rating on the tires that came stock on the vehicle. So I think that it would be a good idea to have an informed discussion of the subject, based on facts, and not based on such things as myths, what so-and-so said, what others want us to believe (in furtherance of their hidden motives), fears of groundless lawsuits, the macho attitude that since one's car can do 140 mph one runs and needs a tire rated for that speed, and hand me down hearsay.

The tire speed rating is the maximum SUSTAINED speed that the tire (not the vehicle that the tire is on) is capable of so long as the load rating of the tire is not exceeded. Sustained speed does not include such things as passing someone or driving for another reason for a short period of time.

The speed rating of a tire has everything to do with how fast you are going to drive, and so long as you do not exceed the speed rating of the tire (and the tire's load rating) you do not have to run tires that match the top sustained speed that the vehicle is capable of. For example, yes, if a Corvette or Porsche is capable of 160 mph, and you are going to drive that fast for SUSTAINED periods of time, its highly advisable to run a W rated tire, which is rated for 168 mph. However, if you are going to stick to the posted speed limits (and as far as I know that's 75 mph or less throughout the U.S.) or do not go more than, say, 20 mph above the posted speed limits, then you can safely put an S (rated for 112 mph) tire on the Corvette or Porsche. You'll still be able to run the Corvette or Porsche up to 112 mph for short periods of time. You'll even be able to run the car at 112 mph for sustained periods of time, because the tire is built to withstand 112 mph for sustained periods of time. And if the tire is built to go 112 mph for sustained periods of time there is obviously a safety factor built in and its good for a higher speed than that for at least short periods of time. Now, reasonably speaking, who among us is going to drive two lane highways and freeways at more than 95 mph for sustained periods of time? Or 105 mph for sustained periods of time, which is still 12 mph under the S tire's 112 mph sustained speed safety rating? Not to mention that you'll be breaking the law, may get an enormous ticket, and may not be a good enough driver to drive 112 mph for sustained periods of time.

A tire's speed rating takes into account speed during cornering. Obviously, the tire's not always going to be going in a straight line, and will be subjected to cornering. So the cornering ability of a car at speed does not determine the tire rating. Its false to say that a Porsche will take corners faster or quicker than a Forester so a Porsche needs a higher speed rated tire than a Forester. As far as cornering goes, the Porsche only needs tires with a speed rating equal to the top speed at which the Porsche will in fact be cornered. So if you aren't going to be driving down the road at over 95, or 105, mph (or indeed, 112 mph), the S tire, rated at 112 mph, is again more than adequate on the Porsche. As a matter of fact, if you corner the Porsche tight enough and fast enough you'll either roll it or spin out long before you hit over 112 mph and be subject to possbile tire failure because you cornered at a greater speed than the tire is rated for.

The higher the speed rating, the more expensive the tire, because the tire must be built better to withstand the higher speed. The higher the speed rating the harder (as against softer) the ride, because of the sturdier build of the tire. So those who buy tires with a speed rating way above the speed at which they'll drive are wasting their money and sacrificing softness of ride.

My state has no law requiring a tire to match the vehicle manufacturer's speed rating or the speed rating of the tires that the vehicle came stock with. If your state does, I'd like to see an official citation to that law or a post with the law reproduced in the post, because I strongly suspect that claims of laws governing this are based on myth or are untrue hearsay.

As to liability issues and fears of insurance company claims, since the speed a tire's run at, not the vehicle's top speed, determines the tire's speed rating, most of the claimed concerns over such things as lawsuits and insurance company denials are bogus, so long as you were driving at or under the tire's speed rating. I know, I'm a lawyer. If you had a tire-related accident while driving your Porsche at 100 mph with a 112 mph rated S tire you were well within the tire's speed rating and the fact that the Porsche would do 160 or more at top speed is irrelevant. In fact, if you were going 100 mph in the U.S. with a tire of ANY speed rating your speed and recklessness are what's going to get you into trouble with the lawyers and the insurance companies, not the speed rating of your S rated 112 mph tires. In addition, we all know that an insurance company often uses any excuse to refuse to pay and you can't do anything anymore without someone saying that it will subject you to a liability lawsuit. Who of us lets the constantly asserted and largely groundless threat of liability suits determine all aspects of our life?

If a manufacturer recommends a particular tire speed rating its probably because the car is capable of a sustained speed equal to that speed rating. Yes, they are probably trying to cover themselves, because they know that many people will drive at the 120, or 140, or 160 mph that the car's capable of and may do so with tires that aren't rated for that speed. But if that happens, the manufacturer is not going to be liable. It wasn't the car's fault. It was the fault of the driver who drove above the tire's speed rating. There is nothing that requires you to run tires equal to the manufacturer's recommended speed rating. Doing so when you won't be going that fast costs you money and ride softness and plays into the hands of manufacturers who are trying to use you and your money to insullate themselves from their fear of groundless lawsuits.

Speed ratings were first developed as a response to the fact that people were driving Germany's autobahn's at very high rates of speed -- 120, 140, and more, mph. The speed ratings let people know that they could buy tires that were safe for sustained driving at a particular high speed. If a person never drives the autobahn at, say, more than 112 mph, then they are ok with an S rated tire, even in a Porsche, and they are also ok with that tire if they don't drive faster than 112 mph on the U.S. highways. An H rated 130 mph tire (which is assertedly what Subaru recommends on and puts on the Forester) is overkill on American roads for those of us who drive our Foresters at sustained speeds at or below the speed limit or even no more than 10-20 mph over the speed limit.

 
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post #2 of 4 (permalink) Old 03-31-2006, 01:17 PM
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Good points but don't forget to account for the increased cornering performance of a higher rated tire. Not just for sustained speeds but the increase in sidewall stiffness that a higher rated tire provides.

Also headroom plays a big part so matching speed ratings close to the sustained speed you're thinking of isn't a wise idea.


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post #3 of 4 (permalink) Old 03-31-2006, 08:43 PM
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It's more of the tire's ability to handle its weight loading with respect to heat. You've also got to bump the tire pressures substantially if you're going to run high sustained speeds.

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post #4 of 4 (permalink) Old 04-02-2006, 04:18 PM
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Here (NSW, Australia) you are allowed to fit a lower speed tyres, but there is a min amount for passenger cars and a different (lower) limit for four wheel drive (Forester counts as that)
The only technicality is that you have to fit a sticker that the driver can see to warn them that it has lower speed rated tyres.

Parts of Australia have no speed limit, so you can legally do 200+km/h

Right now I have S rated tyres on the car (put them on for a weekend of offroading, they will be swapped in a few hours)

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