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Friday, October 1, 2021

What "Size" Tires do you have Part 2?

Last week in Part 1 I covered Passenger car type / size tires. This week we move on to LT and ST type.

The next type tire is “LT” or Light Truck. These will be found on both Class-B and Class-C and possibly a few small Class-A RVs. Since these RVs are larger and heavier vehicles, they will normally have tires that come in larger physical size and stronger Load Range. For example a popular tire for Class-C motorhome might be an LT225/75R16 115/112 LR-E. The double number 115/112 is the different Load Index for single (front) application and dual (rear) application where two tires are mounted side by side. Note that some RV trailers may have two axles, one in front of the other, this is not called “dual” but would be a “Tandem axle” and the tire loading is considered “single” when looking up the capacity in the tables. The Load Index number can be applied to a group of tires that have similar but not identical load capacities. When replacing tires, you should not go to a lower Load Index number.

In LT type tires, the abbreviation of LR for Load Range is used. You can think of the Load Range as a replacement for Ply Rating. For RV application you will see that Load Range starts at C (old 6 ply rating) and moves through D, E, F, and G. This is really an indication of the strength of a tire to hold the inflation pressure, not the ability of a tire to support additional load. It is important to remember it is the inflation pressure and not the tire construction that supports the load. This is why we have Load and Inflation tables not Load and tire construction tables. You should not consider moving to a lower Load Range than selected by the RV manufacturer. The LR along with the original size and type tire and minimum inflation is shown on the RV Certification Label. Basically, the Load Range identifies the highest level of cold inflation to be used starting at 50 psi and moving up to 100 psi or even higher as identified on the sidewall of your tires. Like Speed Symbol you should never consider moving to a lower Load Range than selected by the RV manufacturer.

Do you know where your Certification Label is for your RV? If not, you can contact your RV dealer or post the question on almost any RV Forum on Facebook. The tires on this RV are Load Range C. 



Once you find the label / sticker, I suggest you take a picture as there is important safety information the label that you should keep along with other important papers like your Registration and Insurance card.

Next, we will cover "ST" or Special Trailer tires. This is a special type tire, unique to the US market. It was developed and introduced in the late 60’s for exclusive use on trailers. In fact, it is against safety regulations to use ST type tires on vehicles designed to carry passengers. In this category we might find an ST205/75R15 101K LR-C. For these tires the ST205/75R15 101K is similar in meaning to what we saw in the Passenger type tires. The primary difference is that the Speed symbol K, L, or R is lower for these applications than for passenger type vehicle. The K stands for up to 68 mph. Because the ST type tires are expected to carry higher loads at higher inflation levels the trade-off is the restriction to be operated at lower speeds. The load formula used by tires engineers when designing ST type tires is based on a stated upper operating speed of 65 mph. While today highway speeds can be significantly higher than 65 we should remember that when ST tires were introduced we had a nationwide Speed Limit of 55 mph so a tire design limit of 65 mph was not unreasonable.

While on the topic of Speed Rating people need to understand that the test is designed for Passenger car tires and only requires a new tire be capable of running up to the stated speed for 10 minutes while running a reduced load of 88% of the tire stated max.

You should be aware that in 2002 both P and LT type tires had the DOT test and durability requirements significantly updated and improved by requiring more durability for those tires, but ST tires only need pass the same tests as in 1970. Many consider this increase in performance and durability desirable as it can be considered a measure of “Quality”.

Next time we will cover the heavy duty tires used on Class-A Motorhomes. These include 19.5 and 22.5 rim diameter and a few others.


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