Tire Speed Rating

[mackiee]

HI There,
I am looking to purchase new tired for my Hyundai Elantra GLS 06 (p195/60r15). The manufacturer recommends a speed rating of H. I don't think this car needs an H considering I do not race it or drive it hard. Is it wise to drop the speed rating from H to T? Which is only one speed rating less?

Thanks in advance

[CapriRacer]

First, what you are proposing is a 2 step reduction. There is a "U" between T and H. I'll explain below why you don't find any U speed rated tires in the marketplace.

When car manufacturers specify tires for their vehicle they consider many factors. Obviously top speed and vehicle weight are considerations, but there is more.

As a general rule, the higher the speed rating, the more responsive the tire is to steering input. Generally, T speed rated tires, although more responsive than S rated tires, will not be as responsive as H rated tires. I can almost say without fear of contradiction that even the least responsive H rated tire will be more responsive than the most responsive T rated tire.

This means that you are almost definitely going to lose some of the crispness of the vehicle's handling. In other words, the vehicle will be sluggish in manuevers.

In addition, there is a safety aspect to this that ought to be seriously considered. We engineers tend to think in terms of capability vs risk. In this case we are talking about risk of tire failure vs speed capability. We could also talk about risk of failure vs load carrying capacity.

What is said in engineering circles is that the more capable a product is, the less likely it is to fail. Since tires can be damaged by road debris, punctures, pot hole impacts, cuts, underinflation, etc., they are overdesigned (as is every product), but nevertheless, there still exists a certain amount of risk - and using higher load carrying capacity tires or higher speed rated tires reduces that risk.

Speed ratings are based on a test. The conditions of the test are based on typical European conditions - which means lower temperatures, smoother roads, better maintenance, etc.

It is fairly easy to get a passenger car tire to pass an S rating, and it isn't too much of a stretch to pass a T. But in order to get more speed capability, tires need to have a cap ply - more or less. A cap ply is a circumferentially oriented layer of cords - frequently nylon - that resists the centrifugal forces.

When a cap ply is used, the actual speed capability goes up tremendously - much more than would be indicated by the simple increase in rating from T to H. In fact, there isn't any point in having U speed rated tires - They will easily pass a H rating and maybe even a V, so why bother with U?

This means that an H rated tire is significantly less risky to tire failure than a T speed rated tire - all other things being equal.

Personally, I think H rated tires ought to be the minimum that folks should use on cars (and SUV's, and pickups used on the highway.) Further, if you live on the southern part of the US, particularly AZ, NM, TX, NV, CA, or FL (or anywhere else where it gets hot) then the states ought to require H speed rated tires at a minimum.

I hope you gather that I don't think dropping to a T speed rating is a good idea. You can find plenty of tires with H speed ratings, and you can find one that is affordable and suits your needs.

[alphalanos]

I agree. Its just smarter to go with a better quality tire. The tires are the only thing that touches the road, so skimping on them is crazy.

[Spaz888]

Well, this is one of those subjects that's open to constant debate, much like what tire pressure you should have.

http://www.wheels.ca/Tire%20Talk/article/250663

John Mahler
Special to the Star
May 31, 2008

Q: We took our Subaru Legacy to a tire shop where they recommended and installed Toyo Spectrums. When we picked up the car, the mechanic pointed out that Toyo Spectrums are T-rated tires and that Subaru recommends H-rated. Is this a problem? My main concerns are safety and good tire life.

A: To answer your questions you need to know a bit about speed ratings and what they really mean, so here's the 10-cent course. The term speed rating is a misnomer; it is really more of a test of a tire's stability over time while driving at highway speeds.
All tires generate heat when rolling. Imagine the sidewall flexing as it goes from fully extended at the 12 o'clock position to fully compressed at the 6 o'clock position. At 100 km/h, the tire of a small sedan rotates about 800 times per minute, generating a lot of heat. So as speeds go up, the heat goes up, until eventually the sidewall gives up and goes bang.

There is a measurement standard for the length of time a tire can run at a given speed before failure. This is determined on a test machine in a lab that simulates load on the tire and cornering forces, etc. The tire must run two hours at a sustained speed to get a speed rating.
A T-rated tire can run at sustained speeds of 190 km/h and be safe. An H-rated tire can run at 210 km/h. Since you don't go that fast, why does it matter? Remember that sidewall flex? Flex can be increased when the tire is under-inflated, when the car is more heavily loaded, and when the car goes around a corner. So a heavily loaded car with under-inflated tires going down a twisty road can really punish a tire.
The only thing that can help the tire is to make it stiffer and better able to dissipate heat. The result is a higher speed rating.

The automaker works out the worst-case scenario and chooses a tire with a speed rating that gives even the least-informed driver a safety net. Therefore, your Subaru gets an H-rated tire because that tire is stronger and stiffer than a T-rated tire.

Another advantage of stiffer sidewall, higher-speed rated tires is shorter braking distances and quicker response to steering.
The T-rated tires are not unsafe, but the H-rated tires are what technically belong on the car. The proper Toyo in H-rating is the Toyo Versado. The Spectrum is an excellent tire. T-rated tires will have a longer tread life than H-rated tires and they are less expensive.

Email tire questions to John Mahler at
thetireguy_1 @hotmail.com.
Please include vehicle's make, model and year,
tire brand and size and your name, address and phone number.

[CapriRacer]

Originally Posted by Spaz888

Well, this is one of those subjects that's open to constant debate, much like what tire pressure you should have.

http://www.wheels.ca/Tire%20Talk/article/250663

All I can say is "Wow! Is this guy off base!!"

Let me comment in the same order as I find the errors:

1) Speed Ratings and speed capability are more a function of the tire's ability to withstand centrifugal forces than the ability to withstand heat generation. As proof I offer the fact that higher speed rated tires have more cap plies.

2) When tires fail due to high speed, it is the belt package that fails, not the sidewall.

3) The test for speed rating does not include cornering forces.

4) The tire only has to withstand 1 hour at a given speed to pass the speed rating test.

5) Tire stiffness has very little affect on the temperature generated. Most of a tire's stiffness is caused by the inflation pressure and not the stiffness of the tire itself.

However, I agree with his conclusion that more capability results in a safer situation. It's just a shame that his reponse to the original question has so many technical errors.

[blackm20i]

Generally i buy Z rated tires even though i do not drive 165 mph on the roadway. I feel that if i buy a better rated tire they will be that much better at lower speeds and be safer and more efficient.

['97ventureowner]

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