How to Read Tire Size| Top Guide

Tires are an essential component of any vehicle, and selecting the right tire for your car can improve its performance and safety. Choosing the correct tire size can be daunting, especially if you’re unfamiliar with the terminology used in tire specifications. Understanding how to read tire size is crucial to ensure you get the right tires for your car.

Tire sizes may seem like a jumbled mess of numbers and letters at first glance. But once you know what each means, it becomes easier to identify which size best suits your vehicle’s needs. This article will guide you through understanding tire size codes and help you make informed decisions when purchasing new tires for your car.

How Does Tire Size Work?

The digits and letters marked on the tire sidewall indicate the tire size.

How to Read Tire Size
  • Tire Type
  • Tire Width
  • Aspect Ratio
  • Construction Type
  • Wheel Diameter
  • Load Index
  • Speed Index

Suppose you plan to purchase a 315 70R17 tire and have chosen Toyo Open Country AT3. On examining the tire’s sidewall, you’ll notice it reads,

LT 315 70R17 121 118S:

  • LT: Specify the category of the tire.
  • 315: Specifies the tire’s width.
  • 70: Specifies the tire’s aspect ratio.
  • R: Specifies the tire’s build type.
  • 17: Specifies the tire’s wheel size.
  • 121/118: Shows the tire’s load index.
  • S: Specifies the tire’s speed rating.

Tire Type:

LT 315/70R17  121/118S:

 Tires have different types.

P-Metric: The letter P in P-metric sizes is an abbreviation for “passenger.” Regarding tire sizing, the P-metric system is used primarily for passenger vehicles such as sedans, coupes, and SUVs. This system was first introduced by the Tire and Rim Association back in 1976 and has since been widely adopted throughout the industry.

LT: Light-truck tires, commonly known as LT tires, are designed to withstand the rigors of heavy-duty applications. These tires feature stronger sidewalls and a higher load index than passenger car tires. The added strength is necessary to handle the weight and demands of larger vehicles.

No letter: When buying a new set of tires for your vehicle, you may come across Euro-metric sizes. These sizes are commonly used in Europe and indicate the tire’s width, aspect ratio, and rim diameter. The main difference between Euro-metric and P-metric sizes is their load index and inflation pressure.

Tire Width:

LT 315/70R17  121/118S:

Regarding tires, there’s a lot of information to digest. One of the most important pieces of information on your tire is the three-digit number indicating its width. This measurement is crucial because it affects your tire’s performance on the road.

Aspect Ratio:

LT 315/70R17  121/118S:

One of the most important aspects of a tire’s construction is its sidewall height, an essential parameter defining its profile. This measurement is typically expressed in ratio form, which can confuse many drivers who want to know the real height of their tires. To determine a tire’s actual sidewall height, multiply the ratio by the tire width. For instance, if your car has a 315/70 R22.5 tire size code printed on it, you must multiply 0.70 by 315 mm to get 220.5 mm.

Construction Type:

LT 315/70R17  121/118S:

Two distinct construction types are available in the market, indicated by the letter R on the tires.

  • R: The latest and most efficient building method is radial construction.
  • No letter: One thing to remember when looking at tire sizes is that if you see a size like 7.00-16, this tire has a cross-ply construction. While these tires are still used on some vehicles today, they are not as common as they once were. This is because cross-ply tires have some disadvantages compared to radials.

Wheel Diameter:

LT 315/70R17  121/118S:

When choosing the right tires for your vehicle, the wheel diameter is one of the most crucial factors to consider. The wheel diameter refers to the size of the wheel itself, and it plays a significant role in determining how well your car handles on the road. In general, wheels with larger diameters offer improved stability and handling at high speeds, while smaller wheels provide greater maneuverability and a more responsive driving experience.

Speed Index:

When choosing tires for your vehicle, it’s important to understand the tire’s letter indicating its speed capacity. This letter can be found after the numbers indicating tire size and load index. The speed rating represents the maximum speed that a tire can safely handle. Matching your tire’s speed rating with your vehicle’s top-speed capacity is essential to ensure safe and efficient performance.

Some people might think they can use tires with a lower speed rating if they don’t plan to drive at high speeds. This isn’t recommended as it can compromise your safety on the road. Tires with lower speed ratings may be unable to handle sudden stops or quick maneuvers during emergencies. These tires may have slower steering responses and less cornering stability than higher-rated ones, which could impact handling and control when driving at any speed.

How to Find My Tire Size?

Nowadays, tire manufacturers and online dealers have made it easy for customers to find their required tire sizes by developing various tools, such as digital calculators, to determine the correct size based on a few simple inputs. These tools make purchasing new tires or replacing old ones easier without any confusion or hassle. You only need an internet connection and some basic information about your vehicle. You will get the longest life from your tire when you find the right tire size.

Driver’s Door Jamb:

  • Tire size can be located on the driver’s door.
  • Check two places when opening the door: the Door jamb and the curve where the door sits.

Glove Box Door:

  • Don’t forget to check your glove box; it’s uncommon but worth a shot.
  • The sticker you’re searching for can also be found there.

Gas Tank Hatch Door:

  • The sticker is also on the gas tank hatch door.
  • Ink wipes away when in contact with gas.
  • Stickers become useless over time.

Final Words on How To Read Tire Size:

Tire size is indicated by numbers and letters on the tire sidewall. The first number denotes the tire’s width, while the second represents the aspect ratio or height. The letter after these numbers indicates the construction type (radial or bias-ply), and the following number denotes the wheel diameter. It is important to match your tire size with your vehicle manufacturer’s recommended specifications for optimal performance. When selecting new tires, don’t overlook other factors, such as load index and speed rating.


What is the best way to read tire size?

The best way to read tire size is by looking at the tire’s sidewall. You will find a series of numbers and letters on the sidewall that indicate the tire size. The first set of numbers indicates the tire’s width in millimeters, followed by a slash and the ratio between height and width.

What is the size of a tire?

The tire size varies depending on the type of vehicle it is being used for. Generally, tires come in sizes from 13 to 22 inches in diameter. The tire’s width also varies depending on the type of vehicle and can range from 4 to 12 inches.

What is the diameter of a tire?

The tire’s diameter varies depending on the type of vehicle and size of the tire. Generally, car tires range from 14-22 inches, while light truck tires range from 15-24 inches in diameter. Motorcycle tires can be even smaller, 12-19 inches in diameter.