Tire Information, by Tom Warner

So far many have posted information on the inflation pressures and types of tires various netters use on their coaches. It is obvious that the choice of tires for the GMC motorhome is a complex issue and it is complicated by the fact that most of us think we are experts because we have used tires for so many years on our automobiles. However if we take that attitude with the GMC we are going to get into trouble fast. You cannot compare car tires to ones used for an RV and especially not the GMC. The GMCmotorhome uses TRUCK TIRES. I am a perfect example of the ignorance that surrounded this issue, before this forum and our discussions. I bought a 1976 palmbeach March 1998, tires looked good(Michelins), much bigger and heavier than my car tires, they were the proper tires for the vehicle (glovebox sticker told me that)and fat dumb and happy I started to drive it. I told myself that the tires could be changed later during the winter. After several thousand miles of driving, first tire went and than a catastrophic failure on the second causing over a $1000 damage to the coach (generator door, skirt, air bag and paint). Thats when I started looking for information. If you have never had a really bad blow out you haven't lived. It sounded like a bomb going off in the rear of the coach, complete with fire and smoke. Fire later determined to be from the steel belts hitting on the bogie suspension and producing lots of sparks. wonder what that would have been like on the propane tank side if the tank had been turned on and additionally had a leak? Maybe break the LSR?

Some things I found out:

1. Auto tires have a RECOMMENDED and MAXIMUM cold pressure rating, and TRUCK TIRES have a MINIMUM cold pressure rating to carry MAXIMUM loads. Big difference, and very critical. If you mix up these two concepts you are asking for trouble.

2. On radial truck tires(ours for the GMC motorhome) the number of plies has no meaning! Load range and inflation pressure are the determining factors. In other words the GMC needs E rated tires but if you inflate them incorrectly you only have a D range or less tire.

3. A load range E tire is only a load range E tire if it is inflated to the correct pressure for the load. And that is critical also. Truck tires typically are inflated to carry the present load and this can vary day to day in commercial operations by knowedgable people.

4. GMC rims need Schrader TR570 metal stems and metal sealing caps to hold the correct pressure long term. Alcoa rims come with them mounted.

5. During separate East and West coast GMC motorhome rallys some time ago, 86% of the motorhomes tested had underinflated tires and could not carry the measured weight safely. At those same rallys 61% of the coaches were exceeding their gross vehicle weight. I will make a bet with all of you. If you check your pressures right now ( many of you can't since your hubcaps are in the way and you have to remove them to check), at least one and probably all of your pressures are incorrect for the tires.

6. Highest tire loading appears to be on the right front tire and can reach 3000# if the air suspension is malfunctioning or as some have said they drive in hold! That greatly exceeds the load carrying capacity of all D rated tires.

7. In the case of the GMC motorhome, the tires load is constantly changing because of the independent front and rear suspension. If you don't want to exceed the recommended tire loads, than the system must be working properly.

I am still learning and as I find out more will post.


Truck Industry Information

The following information came from


Cycloid is a manufacturer of automatic tire pressure maintaining systems for trucks. See http://www.cycloid.com/welcome.htm for mer details on their system.

Industry Information on the Effects of Improper Tire Inflation

A Cycloid National Tire Usage Survey indicates that almost 25% of the total truck and trailer tire inventory experiences problems each year. Over 80% of all tire problems experienced by the fleets was a direct result of improper inflation. Industry experts say low inflation causes higher... Fuel Costs

" A 10-degree drop in air temperature will reduce tire pressure by 1 psi. The difference between summer and winter tire pressures can be as much as 8 psi, causing a fuel penalty..."

- Sikorsky "How to get more MPG in the 90's"

For truck tires, a 10 psi drop in tire pressure will increase tire rolling resistance by 2%.

- Goodyear Tire & Rubber

Under-inflation is not only the tire's enemy for durability but is a significant contributor to increased rolling resistance and loss of fuel economy.

- Goodyear Tire & Rubber

Tires under-inflated by 15% will cause fuel penalties of around 2.5%, and a 30% under-inflation will increase fuel consumption by 5%.

- Goodyear Tire & Rubber

Maintaining tire inflation pressure is the most important tire maintenance function. Cost per mile benefits will directly result from good tire pressure control.

- Goodyear Tire & Rubber

Tire Costs

Tire costs are now the second highest operating cost after fuel. Driver costs have fallen to third in operating expenses.

- Dallas Carriers

An industry average of 20% of the annual maintenance budget is spent on tires and tire management programs.

- Cycloid National Tire Usage Survey

Running tires at 20% under-inflation can reduce tire life by up to 50%.

- Goodyear & Michelin

Under-inflation will cause increased tire running temperatures and...leads to premature removal of the tire.

- Goodyear Tire & Rubber

Surveys indicate that consistent proper tire inflation for the trucking industry would increase fleet wear by 17%.

- Michelin

A 20% under-inflated tire will, over the life of the casing, reduce the effective life of that casing by up to 30%.

- Industry Brochure

Tire under-inflation will reduce tire life as follows: 20% pressure reduction causes a 25% tire life reduction, and corresponding figures for 30% under-inflation are a 55% tire life penalty.

- Schrader-Bridgeport

A 100 psi tire run under-inflated at 80 psi will loose 15 - 20% of its life, and damage its further life as a retread candidate.

- Bandag Corporation

Tire Debris

Tire abuse causes rubber debris on the road... Almost all debris comes from dual tires, primarily from the inner tire.

- Tire Retread Bureau

"Zipper Tire Failure - the tire has been operated in an under-inflated condition which weakened the sidewall cables."

- MTD Magazine

Roadside Repairs

The average cost for an emergency roadside assistance call was $775.00.

- Cycloid National Tire Usage Survey

Enviromental Issues

Retreads use 7 gallons of oil to re-manufacture a tire versus 22 gallons for a new tire.

- Tire Retread Bureau

Legal and Liability Concerns

Proper tire inflation would save 25,000 lives each year.