How To Read Tire Size?


Whether you’re looking for a spare or need to replace your tires, it helps to know the tire size of your vehicle. Today, it’s easy to get tires with barely noticeable differences in size but major differences in performance. Knowing your car’s preferred tire size will help you choose the best tires for maximum performance. 

Here are 9 things to look for when reading tire size: 

  1. Know what type of vehicle the tire is made for. 
  2. Read the correct tire width. 
  3. Check the wheel diameter.
  4. Make sure the tire’s aspect ratio is correct.
  5. Read the tire’s construction type. 
  6. Check the load index value. 
  7. Check the tire’s speed rating. 
  8. Check the traction and temperature grades of your tires.
  9. Make sure the tires are DOT rated. 

The rest of this article will walk you through how to read your tire in detail using the Tire Identification Number as a reference. You’ll also find helpful tips on differentiating high-quality tires from low-quality ones. 

1. Know What Type of Vehicle the Tire Is Made For

To find out what vehicle the tire was made for, you’ll need the 12-digit tire identification number (TIN). You can find this in several places, including: 

  • The car manual under the tire section
  • Inside the glove box door
  • In the fuel tank hatch
  • The side door jamb

This pin is also engraved on the tires, so you can use it as a reference if you can’t find the TIN elsewhere. 

The first letter of the TIN will tell you what type of vehicle the tire is made for. P is the most common letter on tires and is used for passenger vehicles. Similarly, LT is used for light trucks, ST for special trailers, and T for temporary or trucks. 

Some tires may not have a letter in front of the identification number. These tires are usually graded according to the Euro-Metric grading system. Always install tires for the correct vehicle type to get the right tire pressure. 

Using a passenger vehicle tire on a trailer will damage the tire and vehicle since these tires aren’t made to handle heavy loads. 

Read: Tire Sidewall Damage: What You Should Know

2. Read the Correct Tire Width

After determining the tire type, you should look at the tire width. This is the three-digit number that comes after the initial letter on the TIN and is measured in millimeters. Tire width is usually the first thing to help you determine the tire’s size.

Tire width measures the length of a straight line drawn between one sidewall to another. It’s crucial to determine the exact tire size, but it’s not the only measurement unit. Most experts can narrow down which tires you require just by looking at the type and width, although you may need other measurement metrics for certain vehicles. 

Always check the tire width of individual tires since some vehicles may require different width tires in the front and rear. 

3. Check the Wheel Diameter

Wheel diameter is used to measure the diameter of the wheel on which the tires fit. 

If you’re confused between wheel diameter and tire width, remember that wheel diameter will always be less than tire width. Never ignore this when reading tires since tires with the same width may have different wheel diameters. 

Also, keep in mind that while tire width is measured in millimeters for accuracy, diameter is usually in inches. It’s common to see tires with a 225-millimeter tire width and 17-inch wheel diameter. 

Read: Why Does Gas Mileage Drop With New Tires?

4. Make Sure the Tire’s Aspect Ratio Is Correct

Aspect ratio is used to measure the ratio of the tire’s cross-section and its width. It’s usually the number that comes after tire width and is expressed as a percentage. For example, an aspect ratio of 70 means the cross-section is 70% of the tire’s width. 

A lower aspect ratio is usually better since it indicates a better tire response time. Vehicles with a lower center of gravity will generally have a smaller aspect ratio, and large 4x4s or SUVs will have bigger aspect ratios. 

Some vehicles can handle tires with varying aspect ratios as long as they come within a certain range, although only tire experts can do this. 

5. Read the Tire’s Construction Type

After the aspect ratio, you’ll notice a letter that is usually R. This represents how the tire is made. R means that the tire is constructed in a radial shape where the internal ply is rotated at a 90-degree angle from the base centerline. This gives it more strength and stability and makes it difficult to puncture. 

Older cars usually had tires that were constructed diagonally due to the lack of development in radial tire design. However, these tires have been phased out, and nearly all tires are constructed radially. 

6. Check the Load Index Value

The load index value will tell you the maximum load the tires can carry safely. Always make sure the vehicle is loaded under this weight. Exceeding the maximum load value will cause your tire to take more pressure than it can handle, which could cause it to burst. 

While the load index will be a value, it’s not necessarily the weight capacity. For example, a load index value of 100 doesn’t mean 100 pounds. Rather, it indicates the number 100 on the load index chart. To find out the maximum load capacity relative to your tire’s load, consult this load index chart

Remember, you are legally required to ensure the car doesn’t exceed the tire’s load capacity. If you land in an accident from exceeding the maximum load capacity, you’ll be liable for fines and damages.

Read: How to Tell if Tires Are Directional?

7. Check the Tire’s Speed Rating

While your car may have the capacity to go over 120 mph (193.12 kph), your tires may not. Going too fast may cause the tires to wear off faster, affecting their traction. The best way to measure how much speed your tires can take is by looking at the tire’s speed rating. 

The tire’s speed rating will usually be a letter rather than a number. These letters represent a certain range in the maximum speed that the tire can handle. For example, S indicates the tires can handle a maximum of 112 mph (180.24 kph), while Z is used for tires that can go over 150 mph (241.40 kph). 

It’s important to remember that the speed index rating is the maximum speed that the tires can handle under maximum load capacity. If the car is lighter than this load capacity, you may go faster without damaging the tires. However, never go above the speed limit to ensure your safety on the road. 

8. Check the Traction and Treadwear Grades of Your Tires

The best tires have excellent traction and a high treadwear grade. Always compare different tires’ traction and tread wear values to help you choose the best quality tires. Traction is used to measure the tire grip under wet conditions, while treadwear indicates the durability of the tires. 

Traction is usually graded in letters which are used to identify the grip rating of the tires. The best tires have AA traction. This indicates that they have a great grip in wet or snowy conditions. While these tires are slightly more costly, they are safer as well, so always choose tires with the highest traction rating. 

Similarly, the treadwear value is used to indicate the expected lifetime of the tires. Tires with a higher treadwear value will last longer and may need less frequent replacements. However, the lifetime of your tires will depend on how you use them, and the treadwear value is just an estimation. 

Read: How To Tell If Dealer Rotated Tires

9. Make Sure the Tires Are DOT Rated

The Department of Transport approves tires based on several metrics, including safety, durability, and traction. Tires with DOT approval are safer and usually fulfill their performance requirements. 

Some tires will come with ECE approval, but this is usually for tires manufactured for use in the European Union. Most tire manufacturers will also indicate that the tires have passed certain safety tests, so check for these approvals when buying new tires. 

Read: Should I Balance My Tires Before An Alignment?

Key Takeaways 

Always remember that tire size is not the only factor to consider when buying new tires. Before choosing new tires for your vehicle, you should compare the traction values, durability, speed and load index, and other factors. 

Also, never exceed the tire’s speed or weight rating and avoid doing burnouts to keep your tire’s performance up to the mark.

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