There is a lot of information stamped on the side of a tire. In this series, we are going to help you decode all that information.
You can see this important information stamped on the side of the tire in the picture above. The size is P205/75R15. Following that is the service description of 97S. Finally, there is an M+S for Mud and Snow, meaning the type of tire.
This time, we’ll talk about the service description. The service description is two separate ratings.
The load index is a numeric value representing the maximum weight the tire can support.
The speed rating is represented by a letter, and it’s the maximum safe operating speed of the tire.
In our example above, the load index is 97 and the speed rating is S.
A tire’s load index is the amount of weight the tire can support. The basic idea is that a higher number equals a higher weight, as long as the tire is at the correct pressure. This is important since overloading your tires will cause them to deteriorate faster, or worse, blow out completely.
The good news is that it’s very unlikely that modern tires will self-destruct, even if you stress them to the maximum. Plus, most manufacturers will recommend tires that are overkill. That way, when you load your car full of people, dogs, camping gear, and a canoe, you’re still under that maximum weight.
Common Load Indexes
You can take a look at this massive chart if you want to know every index rating, but most of them are rare. 72-126 is the range youu will see most often since those cover most vehicles on the road.
This table is simply a quick representation of the most common numbers; refer to the previously linked chart if you need specifics.
Duallies and Alternate Ratings
Some specialized tires will have multiple speed ratings or be following a different standard. Chances are that if you need these special tires, you will know exactly what you need. It’s simply beyond the scope of a single article to cover the thousands of different individual standards that exist.
The exception is tires that are dual rated. Light truck and special tires are often used in dual-wheel applications, which means four wheels per axle. There may be two separate ratings, like 104/101.
The first number, the 104, is for use as a single tire (2 tires per axle). The 101 is for use as a paired tire (4 tires per axle). You use the lower rating for duallies so that there is an extra margin of safety.
Finding Out What Load Index You Need
There are two ways of finding out what load index you should be shopping for. It’s important to know that the calculated rating is simply a minimum for safety. Going over the rating if you expect to haul a lot of stuff or for peace of mind will not cause any problems.
Follow the Manufacturer’s Instructions
The first way to tell is really simple: use whatever your car’s manufacturer recommends. The information is often stamped on the inside of your driver’s door, in your user manual, or can be found online with a quick search of your vehicle and the words “tire specifications.”
The second way to find out what load index is a little more complicated, but it may be necessary if you often carry heavy loads or have a heavily modified vehicle.
First, you have to find out your vehicle’s total weight, including the maximum amount of weight you can load onto it. That’s called the GVWR, or Gross Vehicle Weight Rating. Often you’ll find it on the door panel with the tire recommendations or in the manual.
The issue is that if you have a customized vehicle because that can get really complicated. Check out this long article if you want to know more, but you’ll have to just do a lot of homework to find out the numbers.
Then, simply add 10% for safety, and divide that number by 4, since there are 4 tires in contact with the ground.
A Ford Explorer has a GVWR of 6,160. 6160+(.1×6160))/4=1694. Ford suggests a load index of 98, which is 1653lbs. It is nice when the math works out.
A tire’s speed rating represents how fast the tire can go while remaining safe. Manufacturers use a letter to represent the rating.
In general, the higher the rating, the better the tire can handle heat. Heat is one of the biggest dangers to a tire. Tires have to deal with friction from the road; friction generates heat, and the faster a tire is traveling, the more heat is generated, so a higher rated tire has to handle that heat.
A speed rating of S, like our example above, indicates that the maximum safe operation of the vehicle using that tier is 112mph. You probably won’t instantly crash if you go over. You are simply operating outside of the safe zone and increasing your risk of losing traction.
Although the speed ratings range from A-Z, the most common ratings start at M since it’s very rare to find a car that has a maximum speed of less than 80mph. There are just two things to note when looking at the following chart.
The first is that H is out of place. That’s because H was used before the speed ratings were standardized as the current format.
The other item of note is the letter Z. A Z rating simply means “high-performance,” and the actual speed rating varies by manufacturer. A special variation is the ZR rating. The ZR will appear after the tire’s size instead of as part of the service description, and it designates a tire that is rated to 186mph.
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How It’s Tested
Knowing how companies determine the speed rating can help you make a better decision about what speed rating you need. Les Schwab has a good overview here. Basically, a tire is spun in a lab for a while, and it either passes or fails the test.
The real world is nothing like a lab, though, so even though a wheel can theoretically handle traveling at 150mph, that doesn’t mean it can actually sustain that on the road where temperature and road conditions can greatly affect performance.
Determining What Speed Rating You Need
Compared to the load index, the speed rating you need is more subjective. If you plan on doing a lot of spirited, high-speed driving, you will want a higher speed rating than the factory recommends.
However, if you are very safe and conscious of all rules, it’s unlikely that you will need a tire rated higher than P. Speed limits in the US rarely exceed 75, so a rating of P gives you some room to pass big rigs every so often.
You shouldn’t mix and match tires, but if you are in a situation that forces you to use tires of different speed ratings, the lowest number is your maximum speed.
This Is Part of a Series All About Tires
Our aim is to demystify the tire buying process and help you make the best decisions possible. Feel free to continue learning by jumping over to one of the following topics:
The Uniform Tire Quality Grading System