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Wednesday, April 11, 2012

How fast is safe to drive on your tires?

I found and responded to a forum thread on tire speed ratings and thought I would share what I learned with you here.

I think it best to think of a tire's "maximum speed" a bit like the engine red line. Can you exceed the red line on your engine? Yes, occasionally, but is it good for the long term durability of your engine? I think you will agree that it's not a good policy unless you like blowing up engines (or tires).

A tire has a finite life. Heat and cycles consume that life. Heat comes internally from under inflation, overloading and fast speed and externally from spending all your time in the South where temperatures are higher than in snow country. Cycles are simply miles.

Heat generation occurs inside the tire structure at the molecular level. It is not linear so one mile at 75 mph “uses up” more of that limited life than two miles at 40 mph. It’s even possible that a mile at 80 mph is worse than two miles at 60mph.

The maximum speed rating is also based on the assumption that the tire has NEVER been operated while 'flat." Note that a loss of 20 percent of the minimum inflation needed to carry the load is considered flat.

Also if the tire has ever been repaired, the manufacturer may no longer support the speed rating marked on the tire. You need to consult the manufacturer's publications to confirm its policy. This would apply to tires with a speed symbol letter as found on most Class-B and Class-C motorhomes.

An example might be LT235/85R16 LR-E 120/116Q. The 120/116Q is the service description with the 120 and 116 being the load index for single and dual position. The “Q” is the speed symbol for 99 mph max. A Class A motorhome might display 255/70R22.5 LR-G 138/134L. The “L” is the speed symbol for 75 mph.

As I cover the topic of speed rating in my tire seminars, there are different requirements for tires in Europe than the USA and Canada. In Europe vehicles are capable of higher speeds (100+ mph). Roadways exist to run them, and it’s legal. The tire description system was created to assure replacement tires were also capable of those higher speeds.

However in North America we have speed limits that do not allow driving at excessive speeds. So the emphasis is placed on handling, not "all out" speed.

The same construction features required to achieve higher speed ratings also improve handling. So, in the Americas a speed rating also tends to be a performance or handling rating.

I know of no highway tire that can run its entire life of 40,000 to 90,000 miles at its maximum speed. The tests to establish the speed rating only require an hour or so to complete and most of the test time is well below the maximum. This test is used to measure the rate of heat energy generated with tires failing due to too high an operating temperature.

Finally.
If you tow a trailer that has ST type tires you need to remember that they are rated at 65 mph maximum. A few trailers come with commercial tires that are only rated to 62 mph. So your maximum speed is based on the lowest rating of all your tires.

6 comments:

  1. Gee, and I always thought the "R" stood for Radial.

    ReplyDelete
  2. Well Penny, you are paartially correct. The "R" just before the wheel diameter does mean Radial. If you see a "D" that means "Diagonal" or a bias ply tire which is an older construction.

    ReplyDelete
  3. I'm thoroughly confused by the 'example' quoted in your article. It shows a number, LT235/85R16 LR-E 120/116Q, then breaks it down to 120/116R with the “R” as the speed symbol for 99 mph max. "A class A motorhome might display 255/70R22.5 LR-G 138/134L. The “L” is the speed symbol for 75 mph. Are you saying that the letter R is for 99 mph and the letter L is for 75?

    You further say at the end, "Finally: That ST type tires...are rated at 65 mph maximum". Is there a chart somewhere that lists these symbols and what they mean?

    ReplyDelete
  4. Let me try again.
    LT > Light Truck
    235 > Overall width in mm
    85 > Aspect ratio or distance from rim to tread divided by overall width
    R > Radial
    16 > Rim diameter
    LR-E > Load range "E" (this relates to the max pressure for the tire)
    120 > Load Index for Single application
    116 > Load Index for Dual application
    Q > Speed Symbol (99 mph catigory)

    The larger tire is similar but there are no letters in front of the tire width.

    There is a chart for the Load Index numbers to a number of pounds but not all tires have the Load Index but they do have the Max inflation and the related load capacity molded in small print on the sidewall.

    the "L" at the end is the Speed Symbol that stands for 75 MPH

    RE St type tires. There is no chart as they all have a speed limit of 65 mph. There is no requirement to label those tires with a speed symbol.


    Hope i was a bit clearer this time.

    ReplyDelete
  5. I think the misunderstanding is because you listed the initial example with a speed rating of "Q", and then immediately proceeded to explain what it would mean had it said "R".

    "An example might be LT235/85R16 LR-E 120/116Q. The 120/116R is the service description..."

    I would recommend just changing the text for future people who read this (stick with Q, its less confusing) - and then deleting this comment I am leaving ;)

    ReplyDelete
  6. Bakari Thanks. Good catch.
    Now I understand the confusion. I made the corrections.

    ReplyDelete

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