RV tire types—Part 2: Changing from ST- to LT-type tires

 In Part 1, ST-type versus LT-type tires: Which are “best” for RVs?, we left off with having to do some calculations.

I will assume you have confirmed the actual individual tire loading and have moved some heavy items around to end up with a reasonable balance of loads.

I will also assume you still want to change from ST-type to LT-type tires. That means you must increase the Load Range and/or increase the tire size to get a load capacity in the LT to match or exceed the capacity of the original equipment (OE) ST-type tires.

Load & Inflation tables

If you compare the Load & Inflation tables for ST-type and LT-type in the tire Data books (ST here or LT here), you will see that there are no LT tires of the same size or Load Range as an ST-type that can support the same load. You can confirm this by also noting the “Load Index” which is a pair of numbers like this:

 

The dimensions are obviously 225/75R16 and the Load Index numbers are 115 for single and 112 for dual application. The “R” is the “Speed Rating” for this tire which, for RV tires, is a relative rating of the heat resistance of the tire. Tires in RV application are limited to 75 mph max, except for ST-type tires, which have a max of 65 mph, based on the Load Formula limitations. (I will cover speed in another post.)

Before we move on, you need to realize that “LT” is a designation used here in the U.S. In European and some Asian countries they have what they call “Commercial” tires. These Commercial sizes do not start with LT or CO but will probably look like 7.00R15, or for metric sizes, 205/75R16C. The “C” in this case is not the Load Range but stands for “Commercial”. The Load Range will be identified as normal LR-C, LR-D, etc., or possibly with R or XL for Reinforced or Extra Load. To make this post easier to read, I will limit my comments to LT-type tires. Just remember there are other options that may be better for those with 15″ or 14″ wheels that do not want to change rims.

 

NOTE: All of these letters and numbers are important when selecting a size, so be sure you record them all when doing your research.

So, on to the next step:

Dimensions

There are two key dimensions: Outside Diameter, or OD, and Width. I am confident that we all understand OD, but width can be a bit confusing. Depending on the wheel well contour, the overall maximum width or “Section Width” may be most important. Some tires may have a narrower clearance nearer the tread, so they will need some actual measurements at a number of locations.

It may be easiest to use the dimensions for OD and “Section Width” published for your current tires and just do a confirmation with your tape measure. Remember tire “width” is not the same as tread width.

You need to be sure the tires NEVER contact any portion of the RV frame wheel well or bodywork. This is especially true for the front position of a motor-home but since we are focused on ST to LT, there should be no ST tires ever placed on the steering axle of an RV. You should try to have equal or greater clearance with the new tires than you have on your original size, if at all possible.

The challenge

When moving from ST-type to LT-type, you will need to move up in Load Range or up in Size, or both.

Now comes the research to see what your options are

Knowing the target Load Capacity and the maximum OD and Section Width, it’s time to use the Internet to do some research. The objective is to find tires that meet your needs for the numbers and that have an appropriate tread pattern. You certainly don’t need Snow Tires or heavy traction tread pattern for normal RV use. I would suggest that the tread be identified for “All Position” or Steer for your trailer application. I run “HT” type tires which stands for Highway Traction.

You can go to websites from large dealers such as Tire Rack, Pep Boys, Walmart, NTB, Discount Tire, or similar. You might also just Google “Trailer Tire” + the name of a large city or town near your location. Once on their website, find the various possible tires that meet your needs.

Next, confirm inflation

If you are increasing the Load Range with the associated increase in inflation, you need to confirm the wheel can manage that higher inflation. The info may be marked on the back side of the wheel, or you may need to contact the wheel seller or manufacturer. Or you may need to get different wheels if your original equipment seller doesn’t know what the rating is for the OE wheels.

 Consider tire warranty

Finally, as I have previously suggested in any comments on “The Best Trailer Tire,” you need to make your purchase decision not just on lowest price, but you also need to consider the tire warranty, if there is one. Or check if there is a Road Hazard Warranty. Also, look into how easy it will be to get a replacement if your tire gets a sidewall cut or un-repairable puncture.

Last step

After all this work, we want to do a first class job. Some might want to say you can’t change from ST-type to LT-type due to Federal Regulations. Well, that just isn’t true. The Certification Label on your RV provided some MINIMUM STANDARDS for Load Capacity and Inflation. There is nothing wrong with increasing the Load Capacity of your tires.

I hope these two posts have helped you understand the steps, calculations, measurements and research needed to make such a change.