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Friday, April 3, 2015

How fast can I safely drive?

Recently saw a story on AP titled "Big rigs often go faster than tires can handle"

The basic point of the article was that over the past few years, states have increased highway speed limits from the 50 or 60 mph range to 75, 80 or higher but the tires in use are for the most part still rated for 75 mph and below.

I know I read many posts from RV owners stating they travel at speeds well above the max rating for their tires. Be that exceeding the 65 mph limit on ST type tires or exceeding the 75 mph rating whenever in RV service.

Some people incorrectly believe that since they drove faster than the tire speed capability last week and didn't have a problem they can drive that fast whenever they want. Nothing is further from the truth.

As I have pointed out in numerous posts both in this Blog and on various RV forums, tires can tolerate a certain level of abuse be it over-load, under inflation or over speed. But every minute a tire is operating outside its design capacity the driver is consuming large portions of the tire life and the tire will eventually fail. Sometimes in a catastrophic manner.
When that happens all to often the driver says "I was just driving 50 mph down the road and had checked the air that morning when the tire blew out. must have been a defective tire'. In reality it was the driver's conscious decision to ignore the specifications for the tire be it load, inflation or speed and many miles of improper operation finally caught up with the driver.

If you take a look at the Tire Selector link for Goodyear RV tire application and download the RV Tire Care Guide on pg 16 you clearly see the statement of 75mph max speed.

Checking Michelin web site we can see a similar 75 mph max as seen on THIS sample pg.

Firestone says 75mph as seen here.  and Bridgestone on their R250 says 75mph
and Toyo has a similar limit for truck/bus type tires.

The important thing to remember is that just like an engine Tachometer that has a "Red Line" engine rpm limit, it may be possible to exceed that limit for a short time but you will pay for that practice in shorter engine life or in the case of a tire shorter tire life.

Couple of comments. When using the above links you might need to do some additional searching as some web sites are portals to additional information.

Some tires such as LT type may have a higher speed rating than 75 but that would be for normal pick-up truck service and not for RV service which is different. Also ST type trailer tires have a lower speed limit of 65 mph and in some cased certain heavy duty trailer tires have even lower 62 mph limit.

I would strongly suggest you lower your actual normal travel speed to 5 mph lower than the tire limit and of course never exceed the posted limit or the limit that could be considered safe for the conditions.

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  1. In speaking with my tire guy and he stating he spoke with his distributors, he says he can find nothing stating 65 mph max for ST tires. I no longer run ST tires but LT and seldom exceed 60 mph but where can I find "proof" to take to my tire guy that there is limits for ST tires? Tks

    1. "Speak with" sounds like a game of legal telephone. Go directly to the manufacturer's tech specs, and if you can't find the info, ask the manufacturer's customer service about the full specs for your tires.

    2. First, the 65 mph speed limitation is part of the tire standard as published by the Rubber Manufacturers Association. By definition, an ST tire has this - except where specifically stated otherwise.

      I do not know why this limitation isn't written on the sidewall of some tires - but it isn't.

      And lastly, it isn't important that your tire guy know of the speed limitation - but it is important YOU do!


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