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.