How to Read Tire Size (What Sidewall Numbers Really Mean)

Trying to understand the confusing letters, numbers, and codes on your tires? Decoding sidewall numbers including tire size is a lot easier than you may think.

Keep reading to become fluent in the language of sidewalls by learning to interpret tire sizing, safety grades, composition and speed ratings.

how to read tire size

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Reading Tire Numbers and Sizes

When someone needs to purchase a new tire, they’ll usually go to a tire shop or dealership and have them determine the size of the replacement tire needed.

But you can save a lot of money and get exactly the tires you want by figuring out this information yourself. All it takes is for you to understand the markings on the sides of the tires. And don’t forget the space saver tire in the trunk. Below are 5 proper ways to read the tire size.

See Also: What Do the Numbers on Motor Oil Mean?

Tire Type

wheel spacers on truck

Before you worry about the specific size measurements of the tire, you need to first make sure it is the right type of tire. After all, there are tires made specifically for passenger cars, light trucks, and so on.

To find out which type of tire you have, look at the side wall of the tire. You should see a series of numbers which start with one or two letters. If the numbers start with a P, then it means that the tire is for passenger cars. If it starts with an “LT,” the tire is for light trucks.

Temporary spares (commonly referred to as donut spares) have similar marking but are designated with a “T” before the tire size.

See Also: How Long You Can Drive a Spare (Donut, Full-Size, Run-Flat)

Tire Width

wheel spacers warranty

Some tires will list the width in millimeters and others will list it in inches. If you see three numbers on the tire’s sidewall coming after the previous letter mentioned, then these numbers represent the total millimeters of the tire’s width.

If you see two numbers after the letter, then it is in inches. The width is measured from one side of the tire straight to the other side.

Keep in mind that while they may provide more traction and look aggressive, wider tires will affect your MPG.

Aspect Ratio

Nissan 370z Nismo

After the tire width, there will be a forward slash followed by an aspect ratio number. The aspect ratio is another way of saying the ratio of the width of the sidewall to the height of it. The ratio number is the percentage of the width which determines the height.

For example, if the aspect ratio is 60, then 60% of the width of the tire will equal the height. The larger the aspect ratio, the taller/higher the tire’s sidewall. When you see sporty cars with what look like skinny tires from the side, these are commonly referred to as “low profile” tires and these have a low aspect ratio.

Related: Are Wheel Spacers Safe? (and Their Pros/Cons)

Tire Construction

tires out of balance

The double-digit aspect ratio number is often followed by a letter. This letter represents the type of tire construction. The most common letter found here is “R” which stands for radial. A professional auto mechanic will understand what the letter here means.

Radial is the most common construction technique used for making tires. Some other less common letters you might find here are “B” for belted bias or “D” for diagonal bias.

Related:  5 Causes of Inside Tire Wear

Wheel Diameter

GMC Sierra 2500HD Duramax Diesel

The number listed next is the wheel diameter size. This number will most likely be in inches and it determines the size of the wheel that can fit inside the tire. For example, if you see the number 18 following an “R”, then it means the tire can fit any wheel that is 18 inches in diameter.

See Also: Best Snow Chains (for 16, 17, 18, and 19-inch Tires)

Load Index

The next part of the tire size code is the load index, which is a numerical value that represents the maximum load-carrying capacity of the tire. The load index typically ranges from 75 (equivalent to 853 lbs.) to 125 (equivalent to 3,638 lbs.) but can be lower or higher than that range.

The Load Index chart below can help you find the equivalent weight for a specific index value. Remember, it is crucial to follow the recommended load index for your vehicle to ensure maximum safety and performance.

Load Index Chart

Load IndexWeight CapacityLoad IndexWeight CapacityLoad IndexWeight CapacityLoad IndexWeight Capacity
70739 lbs901,323 lbs1102,337 lbs1304,189 lbs
71761 lbs911,356 lbs1112,403 lbs1314,289 lbs
72783 lbs921,389 lbs1122,469 lbs1324,409 lbs
73805 lbs931,433 lbs1132,535 lbs1334,541 lbs
74827 lbs941,477 lbs1142,601 lbs1344,674 lbs
75852 lbs951,521 lbs1152,679 lbs1354,806 lbs
76882 lbs961,565 lbs1162,756 lbs1364,938 lbs
77908 lbs971,609 lbs1172,833 lbs1375,071 lbs
78937 lbs981,653 lbs1182,910 lbs1385,203 lbs
79963 lbs991,709 lbs1192,998 lbs1395,357 lbs
80992 lbs1001,764 lbs1203,086 lbs1405,512 lbs
811,019 lbs1011,819 lbs1213,197 lbs1415,677 lbs
821,047 lbs1021,874 lbs1223,307 lbs1425,842 lbs
831,074 lbs1031,929 lbs1233,417 lbs1436,008 lbs
841,102 lbs1041,984 lbs1243,527 lbs1446,173 lbs
851,135 lbs1052,039 lbs1253,638 lbs1456,393 lbs
861,168 lbs1062,094 lbs1263,748 lbs1466,614 lbs
871,201 lbs1072,149 lbs1273,858 lbs1476,779 lbs
881,235 lbs1082,205 lbs1283,968 lbs1486,844 lbs
891,279 lbs1092,271 lbs1294,079 lbs1497,165 lbs

Speed Rating

Lastly, the speed rating is represented by a letter and indicates the maximum speed at which the tire can carry a load corresponding to its load index. Speed ratings range from L (75 mph) to Y (186 mph), with Z being used for tires rated above 149 mph.

It’s essential to choose a tire with a speed rating that matches or exceeds the speed capabilities of your vehicle, as specified by the manufacturer. Always consult your vehicle’s owner’s manual or tire manufacturer for recommended tire size, load index, and speed rating information.

Speed Rating Chart

Speed RatingMaximum SpeedSpeed RatingMaximum Speed
L75 mphU124 mph
M81 mphH130 mph
N87 mphV149 mph
P93 mphZ149+ mph
Q99 mphW168 mph
R106 mphY186 mph
S112 mph(Y)186+ mph
T118 mph

Tire Notation EXAMPLE

Most tires use metric tire notation, which provides you with tire width, aspect ratio, wheel diameter, and more. Using the above information, here’s an example deciphering a tire that’s sized P205/60R17 82S.

  • Tire type: The “P” before the tire width indicates this is a tire for passenger cars.
  • Tire width: The first number (205) represents the tire width in millimeters. In this case, it’s 205mm or about 8.1 inches.
  • Aspect ratio: The next number (60) is the aspect ratio, which is the percentage of the tire’s height to its width. This tells you that the tire’s height is 60% of 205mm.
  • Tire construction: The letter “R” indicates radial construction, which is the most common type of tire construction.
  • Wheel diameter: The following number (17) represents the wheel diameter in inches. So, this tire fits a 17-inch wheel.
  • Load index: The last number (82) is the load index which indicates the tire’s load carrying capacity. This tire has a load index of 1,047 pounds (using the chart in the section above).
  • Speed rating: The last letter (S) indicates the code for the tire’s maximum speed rating. In this case, that equals 112 MPH (using the chart in the section above).

Importance of Proper Tire Pressure

Checking your tires’ pressure regularly (at least monthly) is super important. It is best to perform this when they are cold (not immediately after driving), and especially before taking a long trip. The correct tire pressure improves your vehicle’s safety, fuel economy, and braking distance.

To find the recommended tire pressure for your car, refer to the Tire and Loading Information Label typically located on the driver’s side door edge or post or consult your car owner’s manual. Remember, the correct tire pressure is based on your vehicle manufacturer’s recommendation, not the numbers on the tire itself.

Custom Wheel and Tire Size Upgrades

Upgrading your vehicle’s stock wheels and tires can improve its performance and appearance. But it’s essential to choose the right size when upgrading. Here’s what you should know:

  • Width: A wider tire can provide more traction, but make sure the new tire width fits your vehicle’s rims and wheel wells without causing any rubbing issues.
  • Diameter: Increasing the diameter of your tires may require a larger rim size, as well. This upgrade can improve your vehicle’s stability, but remember that larger tire and wheel combinations add weight and may affect your ride quality.
  • Sidewall: A lower aspect ratio results in a shorter tire sidewall, which can improve handling and responsiveness. However, be cautious as this may also lead to a harsher ride quality.

When upgrading your wheels and tires, always consult with a professional, and make sure the custom size is compatible with your vehicle’s specifications.

Mark Stevens

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