What Do the Numbers on My Tires Mean? How to Read Tire Sizes
Being able to quickly glance at the sidewall of your tire and determine the tire size your specific vehicle needs can save you time, trouble and money. Even still, not many people know how to actually read a tire size even though it’s printed right there on the sidewall. As a mix of letters and numbers, it’s not as easy to read as finding a size 9 shoe.
Why Tire Size Matters
Other than the fact that tires need to physically fit on your vehicle, there are several reasons why having the correct tire size is necessary.
Your tires support the entire weight of your vehicle and are the only parts of your car that actually make contact with the road. Making sure that you select the proper tire size when it’s time to shop for new tires is critical to the health of your tires and vehicle.
A tire’s job is to carry your vehicle and guide it where it needs to go, so when a tire is too large or too small, not only is performance hindered, but your safety is also put at risk. Your tires need to be wide, strong and thick enough to carry the weight of your vehicle while still being able to turn and move. Scratching along the sides or getting caught while trying to turn could cause serious damage to your tires, forcing you to replace them much sooner than you’d otherwise need.
What Do the Numbers on My Tires Mean?
Looking at tire size for the first time can feel like reading a foreign language, but once you learn how to break down what each section of the tire size means, you’ll find they’re not all that complicated.
Tire type refers to the service or type of vehicle that the tire was designed for. “P” stands for “passenger” and is the most common type of tire. “LT” (light trucks that carry heavy loads) is another common tire type designation that you’ll likely run into. Other tire types include “T” (temporary), “ST” (trailer) and “C” (commercial). If there is no letter, that usually indicates that the tire was manufactured in Europe for European cars. These types of tires are called metric or Euro-metric tires and are similar in many ways to a passenger tire.
Measured in millimeters, this is the distance from sidewall to sidewall across the width of your tire.
Aspect ratio refers to the relationship between the tire’s sidewall height and the tire’s width in the form of a percentage. Sidewall height can be determined by measuring the distance between the edge of the rim and the top of the tire at the edge of the tread. Aspect ratio translates to how well your vehicle can handle the tire. The higher the aspect ratio, the longer it takes for your tire to respond to a turning command, making lower aspect tires often easier to command.
This refers to how the tire was constructed. Typically, you’ll only see “R” for “radial tire” since that is the most common type of tire produced nowadays. Radial tires are constructed by assembling piles perpendicular to the direction that they travel, allowing for a smoother ride, longer tread life and reduced fuel production.
Wheel Size/Rim Diameter
Measured across the center of the tire from one end to the other, wheel size refers to the diameter of your wheel and is typically measured in inches. Overall wheel size is often indicated by the rim diameter as a shorthand for what size tire your vehicle needs.
Load index is a shorthand way of describing the total weight that the tire can handle when it is fully inflated. The higher the load index, the more weight the tire can carry. Goodyear’s tire load index shows how the load index number correlates to the weight the tire can carry. For example, a load index of 81 means that the tire can carry up to 1,019 pounds. Load index for the average vehicle ranges from 75-100, but some tires can range all the way up to 150, which equates to 7,385 pounds.
Like load index, speed rating is another shorthand way to reference how fast the tire is designed to go and is indicated by a letter. Firestone’s chart outlines how each letter corresponds to a speed rating. For example, a speed rating of “U” means that the tire is designed to go up to 124 miles per hour.
How to Read Tire Sizes
Breaking down a tire size measurement into its individual parts makes it easy to read once you’re in the know. Now that you know what each element of tire size means, we can walk through an example of how to read a tire size off of a sidewall.
Example: P255/45R18 103H
P (Tire Type): This tire is a passenger tire, one of the most common types of tires.
255 (Tire Width): The tire width is 255 millimeters from sidewall to sidewall or just over ten inches.
45 (Aspect Ratio): The tire’s sidewall is 45% of the tire’s width, which means the sidewall is about 114 millimeters or just over 4 inches.
R (Construction Type): Radial tire, the most common type of tire produced today.
18 (Wheel Size/Rim Diameter): Going from edge to edge, the tire’s diameter is 18 inches across.
103 (Load Index): A load index of 103 means this tire can carry up to 1,929 lbs.
H (Speed Rating): A speed rating of “H” means that this tire is designed to withstand speeds up to 130 miles per hour.
In summary, a P255/45R18 103H tire would be a passenger tire with a width of 255 millimeters, a thickness or aspect ratio equal to 45% of the tire height which here is 114 millimeters. It is also a radially-constructed tire with a diameter of 18 inches that can carry up to 1,929 pounds and can go up to 130 mph.
We’ll Help You Decode All Your Tire Needs
If reading tire sizes still seems like a mystery, drop by your local Virginia Tire & Auto and let our tire experts walk you through the process. We’ll help you find the exact tire your vehicle needs. Schedule an appointment online or call 855.425.3677 to talk to a tire expert.