Belt size, shoe size, waist size, HVAC filter size, kids’ uniform size, wife’s ring size (maybe). If you ask us, there are far too many numbers that we are asked to constantly maintain in our heads. So it’s no surprise that people rarely know – let alone ever ask – what do the numbers on a tire mean?
We’ve all seen them, but we trust our local mechanics or dealers to know what they mean and ensure they are using them to put the right tires on our vehicles. But as you’ve heard us advocate many times on this site, tire knowledge is tire power, and better information keeps our tires in tip-top shape and, by default, those who ride on top of them safer. In this case, knowing a little something about those numbers on your tire’s sidewall can also save you some money when it comes time to replace them. More on that in a below.
The numbers on the sides of tires tell you information about the tire’s size, age, speed capacity, construction, and quality. They let consumers know exactly which tires will fit their wheels, carry their weight properly, work best in wet conditions and support certain driving speeds.
Let’s take a look at each of the main numbers so that you know exactly what you’re looking at the next time you’re at trivia night at the pub.
The Meaning Behind the Main Numbers on Your Tire
To start, let’s look at the most important numbers to know about:
Tire Width
This three-digit number measures the part of your tire that touches the road. It measures in millimeters how wide your tire is from sidewall to sidewall.
Aspect Ratio
Sometimes called the tire’s profile, this two digit number measures the tire’s height, or sidewall. But unlike the tire’s width, this number isn’t shown in millimeters, it’s shown as a percentage of the tire’s width. So, in this case, the 65 means that the height of the tire – the part from the rim to the tread – is 65% of the tire’s width. The bigger the number, the bigger the sidewall height.
Construction
This letter – which nowadays will nearly always be “R” – tells us how the tire is made. R stands for Radial, which means that the layers of materials used to make the tire (including rubber, steel, polyester) run radially, so perpendicular, across the part of the tire that touches the road. You may occasionally see a “D” which means the radials are diagonal or an “F” which means it’s a run flat tire (want to know what run flats are and if you need them? Check out our article on run flat tires).
Wheel Diameter
Measured in inches (because, sure, let’s mix measurements to keep things simple), this number shows the size of the wheel the tire will fit on.
Now let’s take a look at two other numbers of marginal importance (don’t get us wrong, all of these numbers are important, but whether you’re just interested in learning more or using this article as a resource for your own tire buying needs, it’s important to think of these numbers in different buckets of importance:
Load Index
This number shows how much weight a tire can handle. The number itself is not shown as a certain poundage or percentage. Instead, it corresponds to a common load index chart. Rather than place one here and muck up the page layout, here’s a link to a load index chart on Goodyear’s website. As you’ll see, a load index of 99 means that the tire in this picture can support up to 1,709 pounds when properly inflated. Note: there are different load indexes for different types of vehicles. The Goodyear site has links to other indexes if you are interested in going down that rabbit hole.
Speed Rating
This one is exactly what it sounds like: the top speed a tire can go and still perform as rated and without compromise. Measured in letters A through Y, with A being a top speed rating of 3mph and Y being 186mph.
Other Numbers to Know
The numbers and descriptions above help to decode the large, long-division-looking equation on the side of tires, but there are other important numbers on the sides of tires as well that speak to the quality of the tire. Courtesy of a system called the Uniform Tire Quality Grading, these are a bit simpler to follow since their purpose is written right on most tires:
- Traction Grades indicate how well the tire will perform in wet conditions. Grades fall on a scale with AA being best; A and B in the middle; and C as worst.
- Temperature Grades show how much heat a tire can tolerate, which comes in to play when you think about the fact that higher speeds mean more friction which means more heat on the tire. Fortunately, for most drivers and most driving conditions, this is almost superfluous as even the lowest temperature grade is fine for everyday driving. Temperature grades include A, meaning the ability to tolerate the heat generated by going more than 115mph; B, meaning the ability to go between 100mph and 115mph; and C, meaning the ability to go between 85mph and 100mph.
- Treadwear Grades throw a tiny wrench into the simplicity of the Grades system. Unlike the letter grades for Traction and Temperature that are, well, super easy to follow and understand, treadwear grades are a three digit number that estimates how long a tire’s tread will last in relation to other tires. This last part is key as it allows manufacturers some flexibility to determine which tires they are comparing each one against, but the bottom line here is: the higher the number, the longer the tire tread will last.
Save Money on Your Next Tires
Knowing these numbers is one of the best ways to save money when purchasing new tires.
First, of all of the items consumers buy online, tires are surely one of the least purchased items, even though they’re sold on Amazon! But you can’t effectively shop online for new tires if you don’t know what the numbers on the sides of tires means. Now that you do, you can comparison shop prices, brands, quality and more until you find the right ones for your vehicle.
So after you’re done reading this article and educating yourself on what exactly the numbers on tire sidewalls mean, check out this article from our friends at NerdWallet.com who offer tips for saving money when buying tires online.
Second, these numbers can also help you ensure any replacement tires are, indeed, new tires. Here’s how:
- Every tire has a government-mandated, 12-digit Tire Identification Number from the Department of Transportation. The number lets consumers (or the DOT, if they ever need to investigate anything) know exactly where and when a specific tire was made. Forget the first eight numbers – it’s the last four that you really want to pay attention to: the last two are the year the tire was made while the first two are the week of that year. So 4515 means 45th week of year 2015.
We’re not saying that dealers or mechanics would intentionally try to pass old or older tires as new, but sometimes tires sit in inventory for quite awhile until someone purchases an exact set. Even unused tires that are years old suffer degradation and won’t last as long as newer tires, so be sure to keep an eye on these numbers whenever you purchase new tires.
What Tire Numbers to Really Pay Attention To
Right off the bat, all of these numbers are important as they work together to tell a complete story about each tire. But if you’re searching around for the best price – either online or in a store – here is a suggested order of priority to consider:
- Of most importance are are Width, Aspect Ration and Wheel Diameter. The simple reality is that if these numbers don’t match all around, you’re in for a load of trouble.
- Of medium importance are Load Index and Speed Rating. These numbers are largely issues in which you likely won’t have to contend but should be cognizant of in case your vehicle is subject to weight or speed excesses.
- Of lowest importance are Grades and Construction. Don’t get us wrong, you’ll never hear us advocating to skimp on safety, but the grades are all about quality, and since you pay more for quality, they are largely for each consumer to decide on their own.
So the next time you look at those numbers on the sides of your tires you won’t think they’re some mystical algebra equation. And if you are in the market for new tires and are curious if all four tires on your vehicle have to match, be sure to check out our article on if and when all four tires need to be replaced at the same time.