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Before we dive into selecting the right tire size, it’s essential to understand the tire standards that manufacturers follow.

Technical Standards & Regulations

In Europe, the tire standardizing organization is called ETRTO. In the United States, it’s The Tire and Rim Association (TRA), while in Japan, it’s the Japanese Automobile Tire Manufacturers Association (JATMA). Although there may be other organizations, they generally align with one of these three.

When it comes to passenger car tires, TRA designates them by placing the letter “P” in front of the size to indicate conformity to the TRA standard. On the other hand, ETRTO and JATMA do not use any letters, which can sometimes be confusing when determining which standard is being used.

For light truck tires, TRA uses the letters “LT” in front of the tire size, while JATMA places the letters “LT” after the size. ETRTO, however, uses the letter “C” after the size to indicate tires of this type.

In the realm of medium truck tires, ETRTO uses the letter “C” after the size to designate tires designed according to their standard. In contrast, TRA and JATMA do not use any letters in this context.

Now, here’s what’s essential: Tires with the same dimensions (size) may have different maximum load capacities depending on the standard being followed. So, it’s crucial to consider the relevant standard when choosing tires.

When it comes to load capacity designations, “SL” (Standard Load) and “XL” (Extra Load) are used for all passenger car tires, regardless of which standard is being followed. In the ETRTO standard, “Reinforced” is also used as an alternative to “XL.”

Load Range, on the other hand, is specific to TRA. ETRTO and JATMA use the term “Load Index.” While TRA references Load Indexes, it’s important to note that the values provided are approximate.

To sum it up, understanding the tire standards and designations is vital as they play a significant role in determining the right tire size and load capacity. By considering the appropriate standards, you can make an informed decision when selecting tires that meet your vehicle’s requirements.

Here are two great articles if you wanna dive deeper:

Load Range vs Load Index: https://www.discounttire.com/learn/load-range-load-index

Tire Sizing System: https://www.discounttire.com/learn/euro-vs-pmetric-tires

How to Select the Right Tire Size Based on Your Requirements?

When it comes to selecting tire sizes for your vehicle, it’s important to keep in mind that most vehicles offer more than one original equipment size. Let’s take the 2019 Ford F-150 Lariat Crew Cab 4WD as an example. You have three different tire size options: 275/55R20 SL, 275/65R18 Load Range C, or 275/65R18 SL.

Note: I use Toyo Open Country AT3 for an example tire. You can use this link to see tire options for your vehicle and then make a comparison between the sizes:
https://www.discounttire.com/fitment/vehicle

If you don’t know your tire size: https://tireterrain.com/find-tires-by-vin-number/

Let’s focus on comparing the 275/65R18 SL and 275/65R18 Load Range C options for a more detailed comparison. Your decision should be based on your priorities. If you prioritize a smooth ride and better handling, the SL option is the way to go. On the other hand, if you value hauling or towing performance and tire longevity, the Load Range C option would be a better fit.

As a tire engineer, my perspective often revolves around load-carrying capacity. Generally, tires with higher load capabilities tend to provide a stiffer ride. However, there’s a bit of an illusion to consider. When you compare the two tires in the same size, the SL-rated tire may initially appear to have a higher load capability. But here’s the catch: P metric tires, like the SL option, need to be derated by a factor of 1.1 when used on a truck. Crunching the numbers reveals that the SL option actually has a lower maximum load capability. Additionally, it boasts a lower tread depth, which translates to better responsiveness.

The same principles I mentioned earlier also apply when comparing XL and E Load range tires. In summary, if your vehicle allows for Passenger Car or LT (Light Truck) tires, going with a passenger option will provide you with better on-road manners. However, if you find yourself venturing off-road frequently, own a working truck, or regularly engage in towing and hauling heavy loads, then LT tires are generally the better choice for your needs.

Couple of tips for towing;

• Even though the load index of passenger car tires fits, when it comes to towing, sidewall stability is the issue. Hence, LT tires with Load Range C, D, or E provide better stability. 
• Generally, a load index higher than 119 is recommended for towing
• Proper loading is important for even wearing. 60/40 rule should be followed. 60% of the weight should be on the front axle and 40%of weight should be applied rear axle of the trailer. Besides that, try to center load. Start with heavier ones if you gonna stack them up.

To wrap it up, selecting the right tires comes down to choosing the ones that align with your driving preferences and specific requirements.