Previously we talked about the full tire size nomenclature for Passenger and Light Truck applications.
Large RVs usually run what are known as Truck or Bus type tires. With most new applications being of Radial construction we use the acronym TBR. Here are some examples:
255/70R22.5 LR-G 138/134 M
In this example, the various numbers and letters are similar to those for Light Truck except the Load Index numbers and Speed Symbol are optional so may not be marked on the tire sidewall as part of the complete size nomenclature. The Load Index 138/134 and the Speed Symbol "M" make up the Service Description. If this is on your tire please include it when asking a question.
This is an old “Inch” size description with the tire being about 11” wide. This would be a tubeless type tire.
10.00R20 LR-H 146/142 L
This is an old “Inch” size description with the tire being about 10” wide. This would be a tube type tire. Again the Load Index and Speed Symbol are optional
8R19.5 LR-F 124/122 L
This is another old “Inch” size description with the tire being about 8” wide. This would be a tubeless type tire with the example showing the optional Load Index numbers and Speed Symbol.
The two Load index numbers signify that the tire is rated for a higher load in single application and a lower rating when in a dual fitment.
The Speed Symbol for TBR tires are associated with the following MAXIMUM operation speeds.
F = 50 MPH G = 55 MPH J = 62 MPG K = 68 MPH and L = 75 MPH
If no Speed Symbol is marked on your tire you need to consider the tire is rated for a MAXIMUM of 65 unless you can find printed documentation from your tire manufacturer stating some higher speed limit is acceptable.
If you are writing to ask a question about your tires please include the complete nomenclature including Load Index, and Speed Symbol if marked on the tire.
SPECIAL SAFETY NOTE: Maximum Inflation
In July, MOTORHOME magazine published an article on TBR tires. The author offered his personal opinion that it was acceptable to set inflation higher than the Max marked on the sidewall of a truck tire. Exceeding the max cold inflation on the sidewall has serious potential safety consequences. Large TBR tires have been known to explode and personal injuries and even deaths have occurred when proper inflation procedures are not followed. I can find no industry guidelines that allow this procedure of exceeding the maximum inflation when setting the pressure for normal highway application of any tire.
Rims also have a Maximum rating for both Load & Inflation.
Many wheels have these ratings marked on the rim. If you can’t find that information I strongly suggest you contact the manufacturer and obtain the ratings for your rims. Again Serious injury or even death can occur if a rim fails due to damage and overinflating or improper assembly.
Accuride Safety information and Product Literature you can consult.
Alcoa also has product literature online.
Whenever we are talking about setting the inflation in a tire, we are talking about “cold” inflation. This means the tire is at ambient temperature and not warmed from operation or sitting in the sun.
I would suggest the tire not be driven more than two miles in the last hour and not in direct sunlight for at least two hours. When I did my tire cover study – See my post from June 16 on Tire Covers, I saw almost 40° increase in tire temperature in about one hour for a tire in full sunlight.
I would strongly suggest you never attempt to inflate a TBR tire if the tire beads are no longer fully seated against the rim without using an approved Safety Cage. If there is a problem, the tire and wheel can separate and components can easily go through concrete block wall never mind the side of an RV.
Any tire that has lost more than 20% of its rated inflation is considered flat and if it was driven on in a “flat” condition it needs to be dismounted and inspected by trained tire company store technitians. Not a Mom & Pop garage but a store run by a tire manufacturer.