Ply rating or load range: what to look for when choosing tires for hauling
When it comes to ply rating, load range and choosing tires for hauling, things can get a little confusing. These numbers (or letters) are branded on the side of a tire’s sidewall and they identify how much load the tire can safely carry at a specified inflation pressure.
Here’s why it can be confusing: A modern tire with a 10-ply rating doesn’t necessarily have 10 actual plies in it. This goes back to the days when tires were made of cotton. Back then, the ply rating referred to how many layers of cotton had been used in the tire’s construction. The number of plies was used to determine the relative strength of the tire (i.e. the more plies, the more heavy-duty the tire).
These days, ply rating has become a redundant term because most radial passenger tires have just one or two body plies. Light truck tires, including those with heavy-duty ratings (10 ply and above), generally have only two or three plies.
In other words, ply rating is merely a reflection of how strong a tire is in terms of withstanding higher inflation pressures and carrying heavier loads.
The Adoption of Load Ranges
Modern technology has allowed manufacturers to create tires with fewer plies, which caused the industry to adopt load range designations over ply ratings. The information is presented in a slightly different manner depending on the type of tire you’re using. This information can be found on the sidewall of your tire.
Passenger Tires feature specific categories, with most falling in the ‘standard load’ range.
Light Load (branded as LL on the sidewall) has a maximum load pressure of 35 PSI
Standard Load (SL or no branding) also has a maximum load pressure of 35 PSI
Extra Load (XL) has a maximum load pressure of 41 PSI
Light Truck Tires (LT) designate ranges in ascending alphabetical order, with load range ‘B’ representing a 4-ply rating at 35 PSI; C representing a 6-ply rating at 50 PSI, and so on. As you can see, the further down the alphabet you go, the strong the tire will be.
Special Trailer Service Tires
Similar to light truck tires, special trailer (ST) tires are usually available in multiple load ranges and follow the same alphabetical rating system as LT tires.
It’s important to note that load range varies by tire size and inflation pressure. For example, a larger tire with a ‘D’ rating will hold more air and can be rated for a higher load than a smaller tire with the same ‘D’ rating. Also, a given tire size at a higher air pressure results in a higher rated load.
Considering Load Index
Regardless of whether you’re changing tire sizes or switching the type of tire you’re using, it’s essential to ensure the load index listed on the new tire’s sidewall is either equal to, or greater than, the load index of the tire you’re replacing.
ST Tire Blowouts:
When I talk about ST tire blowouts I am not referring to what we just discussed referencing the defective Chinese tires. In a nutshell most trailer tire blowouts are not a result of ‘bad or inferior’ tires, they are a result of one or more of the following conditions.
- Overloaded tires. Every tire manufactured has a load rating based on the tire’s inflation pressure. A tire’s maximum load is the most weight the tire is designed to support at the inflated pressure. ST tires have some of the highest load ratings. This is one of the reasons they are designated for use on trailers. Truck and automobile tires do not have to withstand the weight and stress that is put on trailer tires. The only way to know if a tire is overloaded is to weigh the trailer by individual wheel position. It is quite possible to weigh an axle and be within the axle weight rating, but when the tire positions on each axle end are weighed separately a tire rating can be overloaded.
- Over & under-inflated tires. Another culprit for tire blowouts is over & under-inflated tires. Failure to maintain the correct tire pressure for the load can result in fast tread wear, uneven wear and poor handling which can all lead to tire failure. Remember, the load rating for a tire is only accurate if the tire is properly inflated for the load. Under-inflated tires cause extreme heat buildup that leads to tire failure. Tire manufacturers publish tire load and inflation tables with information on the correct tire inflation pressure for the load. A big reason tires fail is they are not properly inflated for the load. The appearance of the tire can look normal on the outside, but the internal damage is not visible. Tires with internal damage caused by under- inflation can fail catastrophically without warning. One thought is to inflate the tires to the maximum ‘cold’ pressure found on the tires sidewall if you don’t know the exact load on the tires, or the exact weight of the trailer. This should not however serve as an excuse to not weigh the trailer.
- Improper weight distribution. When a manufacturer builds an RV weight distribution is critical. The weight from front- to-back and side-to-side must be carefully considered to avoid having too much tongue weight, too little tongue weight and/or too much weight placed on the trailer’s tires. The manufacturer did its job distributing the weight when the unit was built, now the RV owner must do their job by properly distributing any weight added to the trailer. Some tire overload conditions can be corrected by distributing the weight in the RV, but you still need to weigh the RV by individual wheel position to make sure there is not an overload condition.
What’s the difference between LT and P tires?
‘LT’ are Light Truck Tires which usually are 8 or 10 ply – having 8–10 layers of steel belt (or thicker gauge belts which are of equal strength to said number of layers). This creates a more robust tire which is capable of handling heavier loads on rough terrain.
‘P’ are tires which are more or less designed to operate predominantly on Streets and even terrain. While originally this was intended only for ‘Passenger’ cars – P-Metric tires can be used on Light/Medium Duty Trucks and SUVs. Typically you will see them more widely used in more urbanized locations – where the vehicle spend more time on paved streets then on gravel or off road.
Is a lt and a st tire comparable for 5th wheel
Yes, both special trailer (ST) and light truck (LT) tires can be used on a 5th wheel trailer. They are actually the only two types of tires rated for this use. The reason for this is that tires on a trailer will be subjected to much harsher conditions that require a stiffer sidewall. A condition in which passenger (P) tires are not equipped to handle. Please note there are two different light truck (LT) tires, ones with the LT designation after the wheel diameter (ex. 7.50-16LT) and ones with the LT designation before the aspect ratio (ex. LT275/80-16). The ones with the LT designation after the wheel diameter are the ones that can be used on trailers.
If you are in between a special trailer and light truck tire then try to think of it this way. The light truck tires are made for just that, light trucks but can also be used on trailers due to their sidewall capacity whereas special trailer tires are made specifically for trailers. They will perform better than a light truck tire in my opinion. The only real benefit I see of using a light truck tire is more tread patterns available which won’t really matter when traveling down the open road. They are also more attainable locally.