Tires: speed ratings, proper inflation pressure, speed capability

Question
 I am shoppping around for tires and would like to know what the speed rating means, the ones in my price range have ratings like S,T, and H. What is best? What is worst? Thank you for your time.


Answer
Jen,

First read this:

http://www.tirerack.com/tires/tiretech/general/speed.jsp

You may want to spend some time in Tire Rack's web site looking at other things of interest.

The second thing you need is what originally came on your vehicle.  Look for the placard on your vehicle.  The placard, among many things, tells the original tire size and the proper inflation pressure for that size.  The placard is usually located on a doorpost or in the glove box.

Now let me give you the rest of the story.

Speed ratings are a laboratory test.  Even though they relate well to actual speed capabilities of tires, there are some things that don't actually translate well.

If your vehicle uses an inflation pressure lower than the rated inflation pressure (rated inflated is usually 35 or 36 psi), then the tires actual speed capability is lower.  I've seen as much as 2 steps lower.

Rough roads also reduce the speed capability of a tire.  Perhaps one step lower is a reasonable guess.

I heard a phrase while attending an enginering conference -   Overdesign / Underutilize!  This means that products ought to be designed with additional capability but they should also never be asked to perform near the edges of their capability.  The further away from the capability the less the risk of failure.  (You can't actually reduce the failure risk to zero.)

So what I am suggesting is that you get the highest speed capability you can - added safety!

Plus you shouldn't go below what originally came on the vehicle.  Higher speed rated tires generally handle better, but ride a bit worst, and going lower in speed rated means you'll put on tires that are not in sync with the suspension as the vehicle manufacturer designed it.

Hope this helps.