Tire Maximum Speed Recommendations

[Briarhopper]

A combination of recent posts and my own recent issues forced me to look more seriously at tire specifications. Something I did not realize and have not seen mentioned is that motor home tires and I think truck tires in general have much lower speed recommendations than smaller vehicles and tires.

Glancing at Bridgestone and Michelin it appears a majority of the tires that would suit motor homes have a maximum speed of 75mph or less. Bridgestone's tire data book reads, “Your tire’s actual speed capability may be less since it is affected by factors such as inflation pressure, load, tire condition (including damage), wear, vehicle condition (including alignment), driving conditions, and duration at which the speed is sustained.”

May be common knowledge, but I figure err on the side of safety and post.

It would be interesting to hear if anyone has tires with a higher maximum speed???

[wolfe10]

Stuart,

I guess I would ask from the other end-- do you ever intend to drive your motorhome OVER 75 MPH???

[Briarhopper]

Brett,

Naw (no), 75 is about 10-15 mph above my intended speed. Reason I posted is you see and hear about traveling at higher speeds and we do have speed limits of 75 and over.

More important to the average folk might be as I read it 75mph is the max for new tires, ideal load and inflation as well as a chassis in excellence condition. So, an individual could be exceeding the capability of there tires at a much lower speed.

[wolfe10]

Stuart,

The 75 MPH max speed limit on the tire should be good for the service life of the tire IF a tire is inflated to the proper PSI per tire manufacturer's chart (see below *) which says that:

1. the tire is not overloaded, since weight is ON the tire chart AND

2. the tire is not under inflated.

*The tire manufacturer's chart gives the MINIMUM PSI for a given WEIGHT. Most of us add 5 PSI to that MINIMUM to account for those big Walmart shopping trips, PSI decrease with temperature drop, etc.

[Guest BillAdams]

I frequently travel at 75 should that be the speed limit. That means that at times I may exceed 75 on a downhill stretch but I make every effort to avoid going over 80. That means the answer, in some cases, is that yes, I will be going over 75. I have also been passed by buses on stretches of road in TX where the speed limit is 80. We run Michelin 12R 22.5 tires and the set we had on the front and tag were over 10 years old when we replaced them. They were replaced due to tread wear and not age but we were at a point in the life of the tire where age was also a concern for us.

[tireman9]

I agree with Brett, but would like to expand on the topic a bit.

I think it best to think of a tire "Max speed" a bit like the engine red line. Can you exceed the red line on your engine? Yes, occasionall,y but is it good for the long term durability of your engine to run at the red line? I think you will agree that that is not a good policy.

A tire has a finite life and heat and cycles consume that life. Heat comes from under inflation, overloading and fast speed along with spending all your time in the South.

Heat generation occurs inside the tire structure at the molecular level. It is not linear so 1 mile at 75 “uses up” more of that limited life than 2 miles at 40. It’s even possible that a mile at 80 is worse than two at 60.

The max speed rating is also based on the assumption that the tire has NEVER been operated while “flat”. Note that a loss of 20% 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 manufacturers publications to confirm their policy. This would apply to tires with a Speed Symbol letter as found on most Class-B & Class-C motorhomes and a few Class-A units.

An example might be LT235/85R16 LR-E 116/120Q The 120/116R is the Service Description with the 116 and 120 being the Load Index and the “R” being the Speed Symbol for 99 mph.

Class A might have 255/70R22.5 LR-G 138/134L the “L” is the Speed Symbol for 75 mph.

I know of no highway tire that can run its entire life of 40 to 90,000 miles at its max 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 max.

Finally. If you tow a trailer or dolly that has ST type tires you need to remember they are rated at 65 MPH max and a few trailers come with commercial tires that are only rated to 62 MPH so your max speed is based on the lowest rating of any single tire.

[five]

Another consideration is if you pull a toad it might have a speed restricion. My CRV has a 65 MPH max speed as listed in the owner's manual. I go over that once in a while, either down hill or passing somebody, but do not do it routinely.

[Briarhopper]

I hear you both (Brett and Roger), but I’m not sure you can convince me that a tires capability does not diminish over time. Mathematically, it can’t start at one point and be at the same point when it finishes. Perhaps we can work that one out over a cup of coffee some day.

The original post was more or less to make anyone aware; who was not already, that tire speed is a factor in a tires capability to carry a load, and that motor home tires usually run closer to their max capability (load and speed) than do smaller vehicles.

My Jeep is like an ant as the tires on it could actually carry the weight of 2 Jeeps at 112mph. That’s the tires capability, not to be confused with the Jeeps capability or my driving capability. I count that as safety margin.

To get to the same load capability, I’d need 18 wheels on the motor home and I’d still be running near the max due to the lower speed capability of the larger tires.

So, if your GVW is anywhere near your GVWR, maybe give yourself a little more safety margin and not try to pass Bill.