Tire Weight Ratings Explained
Weight ratings on tires might be the most elusive part about owning a vehicle. Not many people pay attention to these ratings on the sidewall of their tires, nor do they really have the patience for it. But that’s why we’re here—in our tire shop, we make a point of explaining these ratings in the simplest of terms so that people know their options when buying a replacement set of tires. Knowing a tire’s load rating is critical when replacing a tire, especially when switching to a new type of tire or changing the size of the tire.
What Do I Need to Know About Tires?
So let’s take a look at the things you should know about when looking for a new set of tires. The main thing you should be looking at when shopping for a set of tires is the tire’s load rating, but what does this actually mean? This refers to the amount of weight the tire can safely support. If the tire cannot support the vehicle’s weight—if the load rating is insufficient for the vehicle’s weight—the tire can become overheated and subsequently fail. And no one wants a blown-out tire, right? Especially if it can be easily avoided.
So how do we figure out this elusive figure––this “load rating”––for our vehicles? First, take the vehicle’s gross weight and divide it by four. If you can find each axle’s weight, that’s even better. So say your vehicle’s gross weight is 4,000 pounds, each tire should be able to support 1,000 pounds. Keep in mind that this does not account for additional weight, such as passengers, gear, and construction materials like a load of base rock. So you should not select a tire that only meets the minimum weight of your vehicle. You’ll want to find a tire that offers enough extra capacity to accommodate the potential for the higher weight.
It’s also important to never exceed the maximum vehicle load weight, which is the number that can be found on the vehicle tire placard. While it’s clearly not good to exceed the tire’s maximum load rating, it’s equally important to pay attention to the vehicle’s load weight. In each situation, you are risking tire failure. But it’s also important to understand that your vehicle’s load range is more about the air inside your tire. If you’re carrying more weight than is recommended, for example, you’ll need more air and a higher ply rating to hold the air. So bigger tires with a higher ply rating can generally hold more weight.
How to “Read” a Tire
So how do you decipher the combination of letters and numbers on the sidewall of the tire? Well, let’s start at the beginning, with the “P,” the letter that indicates it is a passenger car tire. Directly to the right of the “P” is the nominal width of the tire in millimeters, followed by the ratio of height to width, and then the type of tire construction, generally “R”, which stands for radial, followed by the rim diameter code. To the right then is the load index and speed symbol. This is the number that tells you the tire’s relative load-carrying capability. The higher the number, the greater its load-carrying capacity. Generally, for passenger car tires and light work trucks, the range is from 70 to 110. On the inside circle of the tire, you’ll find the maximum load rating, the maximum permissible inflation pressure, and tire ply composition and materials used.
Putting It All Together
So let’s take a look at a couple of examples—a passenger car tire and a light-truck tire—to better understand how to decipher all of these numbers and letters. We’ll start with the light-truck tire, which will be indicated by an “LT” where you would normally see a “P” if it were a passenger car tire. We’ll use an example of a larger tire: LT295/70R17. Now, this is a big all-terrain tire, which means it should be able to carry lots of weight, right? Well, it depends. The load range is “E,” which means it has a ply rating of 10 and a load pressure of 80 psi, and the speed rating “P,” which means that it has been approved for speeds up to 93 mph under optimal conditions. A “P” speed rating is uncommon and usually means that it is a winter tire or oversized tire. Generally, though, even off-road and winter tires will achieve at least a “Q” rating. The max load for this tire is 3,970 pounds. The truck has a gross weight of 5,400 pounds. So you are left with a load-carrying capability of 10,480 pounds, which is more than the truck is recommended to carry. But how much base rock could you theoretically carry on these tires? We would need the axle weight here to be precise, and the weight of the rock, the temperature, and the psi, among other things. Which is to say there are a lot of considerations to take into account beyond just your tire’s load rating, especially when hauling heavy loads or pulling heavy trailers.
But what about a passenger car tire? We can be more precise with passenger car tires because it’s easier to calculate the load capacity. Say your tires are: P225/70R16 91S. So we know this is a passenger car tire, and the width is 225 millimeters. The aspect ratio, or how tall your tire’s profile is, is 70, which means the tire’s height is 70% of its width. The “R” indicates that it is a radial tire, and the “16” tells us that it has a 16-inch rim diameter. Now is the tricky part: load index. The load index, in this case, is 91. So if you were to consult a load index chart, you’d see how much weight, in pounds, the tire can support when fully inflated. 91 translates to 1,356 pounds. So this tells us that the tire can carry roughly that amount of weight. So 5,424 pounds would be the max weight for this set of tires. So you would want to make sure that your vehicle, when fully loaded, does not exceed this weight. The last thing we see on the tire is the letter “S,” which is the speed rating. This is important because, at speeds over this rating, the tire can fail. In this case, the tire is rated up to 112 mph, so hopefully, you are not going over 112 mph anyway, because that could be unsafe for many other reasons too.
The Right Set of Tires
What does this all come down to? The tire’s ability to withstand pressure—and speed. And many other variables come into play as well. But what’s important is understanding that you have to find tires that can support your car’s Gross Vehicle Weight Rating (GVWR), which can be found in the owner’s manual. Then simply compare that number to the load index chart, where you can see how many pounds each tire can support. If that number is greater than the GVWR, then you should be fine. But, of course, you can just write down the numbers and bring them into the tire shop at McCluskey Chevy in Cincinnati, OH. Or better yet, just bring in your car or truck, and we’ll recommend the perfect set of tires for your vehicle. We’re good at this, and know how to take the headache out of buying new tires.