How to Read Rim and Tire Sizes: All You Need to Know

If you want to find the best possible tires for your car, you have to know their exact size. Yet, before you can find a perfect fit, you first have to decipher those little letters and numbers written on the tires. But don’t worry, learning how to read rim and tire sizes is much easier than it seems.

Once you memorize what each letter and number means, you can easily find your perfect tires. Without further ado, here’s exactly what you need to know.

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Tire Type

The first letter (or two) that you see on your tire’s sidewall represents the class of your car. Most often, you’ll find either P or LT inscribed into it. These letters will tell you what kind of vehicle or service the tires were designed for.

P

These are the most common types of tires that you’ll find on cars. The “P” stands for “Passenger Vehicle,” and you’re likely to have these tires if you drive a minivan. Also, they’re on most SUVs, light-duty pickup trucks, and cars.

Euro-Metric

Frequently found on European cars, minivans, or SUVs, the Euro-Metric size is equivalent to a P-Metric size. However, these tires don’t have a letter designation and can differ in capabilities and load-carrying ratings.

LT

LT stands for Light-Truck, and it can be written as a prefix or suffix. If it’s at the beginning of the sequence as a prefix, you can use the tires on vehicles that carry heavy loads or that tow trailers. What’s more, you can see these letters on SUVs, medium-duty and heavy-duty pickups, as well as full-size van tires.

On the other hand, if the letters LT are in the middle of the sequence, you can put those tires on vans and sport utility vehicles. Also, you can see them on different trucks that have a load capacity of up to one ton.

T

If you see the letter T at the beginning of the sequence, that means your car has a temporary spare. Keep in mind that these were designed for short-term use only and you should change them with regular tires as soon as possible.

ST

You should only put tires that have ST written on them if you’re driving a boat or car utility trailer.

Tire Width

The first group of numbers that follows your tire type is the width, and it’s always represented in millimeters. They’ll tell you how wide your tire is from the inner to the outer sidewall.

Now, if you have the smallest, 5-inch rim size, your tire width would have to be between 155 mm and 185 mm. But if you have 12.50-inch rims, the tires should be at least 305 mm. With that said, the average tire width you’ll find on most passenger cars is 225 mm.

Aspect Ratio of Sidewall

After the tire width number, you’ll see a forward slash symbol, which separates it from the aspect ratio. The aspect ratio or tire profile measurement is a two-digit number, and it’s the ratio of the height of your tire’s cross-section to its width.

For example, if the number after the slash is 45, it means that the height of the tire is equal to 45% of the tire’s width. If you have a lower aspect ratio, your tire has a shorter sidewall and a lower profile.

So if you want to calculate the height of your sidewall properly, convert your tire width to inches and multiply it with the aspect ratio percentage.

Internal Construction

The internal construction maintains your tire’s stability, and there are three different types:

R: The “R” stands for radial, and it’s present in around 98% of all passenger vehicles and cars. With this type, you’ll have better fuel economy, steering control, and longer tread life than with any other type.
D: If instead of an R, you see the letter D, your tire has a bias-ply construction. With it, your tire’s internal body plies crisscross on a diagonal pattern.
B: A belted B construction isn’t as common these days because it makes your tire feel stiffer, and your steering control is sub-par.

Tire and Wheel Diameter

Without measuring your wheel diameter, you can find out how big it is by looking for the number written after the internal construction. It will tell you what rim size fits your tires. For example, if your rims are 15 inches, you’d have to get tires that are the same size.

The smallest wheel diameter is 8 inches, while the biggest can be up to 28 inches. What’s more, you’ll usually see tires with these standard diameters on SUVs, passenger vehicles, vans, minivans, and light-duty trucks.

Unique Wheel Diameters

Some box vans, heavy-duty trucks, and trailers can have unique wheel diameters that are made in half-inch sizes. However, you should never combine these with traditional rims and wheels.

If you go even half an inch over or under, the tires won’t fit, and they’ll be putting constant stress on your transmission and suspension. Also, selling a vehicle with mismatched tires and rims will decrease its value.

Service Description Rating or Load Index

Except for Z-rated tires, all tires need to have a load index written on them. The number indicates the maximum load your tire can support once you inflate it. Since calculating the load index on your own is pretty tricky, I suggest you take a look at this load index chart.

There, you can see how much your car can carry based on the service description rating on your tires. Also, when buying new tires, it’s best to find an index that matches or exceeds your manufacturer’s recommendations.

Speed Rating

The speed rating is the final letter in the sequence, and it tells you what the maximum service speed for your tire is. The most common speed ratings you’ll find are: Q, R, S, T, U, H, and V. If you have a Q rating, you shouldn’t drive over 99 MPH, while V lets you drive at 149 MPH.

However, you can also come across some Z-rated tires on sports cars that have a W or Y rating. With a W rating, your maximum speed should be 168 MPH, while Y tires can go above 186 MPH. If you want to see how fast your tires will let you drive, you can check out this speed rating chart for more information.

Dot Serial Number

Now, some tires might have the letters DOT inscribed into them. If you’ve found the DOT symbol, it means your tires are in compliance with the US Department of Transportation safety guidelines.

Below or next to the symbol, you’ll find a group of numbers that can help you determine how old your tire is. In fact, you’ll see three or four numbers that will show you when your tire was manufactured.

If it’s three numbers, the tire was made before 2000, with the first two numbers showing exactly in which week of a year it was made.

However, if your tire is from the new millennium, you’ll see a four-digit number. Again, the first two numbers show the week, while the last two reflect the year. For example, if you see 2421, the tire was made in the 24th week of 2021.

Other Information You Could Find

Depending on when and where you got your tires, you could also see some of the following information on them:

Maximum air pressure: This number will show you how much air pressure your tires can handle, and it’s measured in PSI.
Treadwear rating: Next to the word treadwear, you’ll see a number that’s most often 120 or 180. It will tell you how durable your tire’s tread is compared to the industry standard, which is 100.
Traction rating: A tire’s stopping ability or traction rate is shown as letter grades: AA, A, B, or C. The grade AA is the highest and shows the best performance, while C is the lowest.
Temperature rating: From best to worst, your tire’s temperature rating can be A, B, or C, and it shows how much heat it can withstand.

Measuring Rim Sizes

When measuring your rims, you’ll need to look at the:

Diameter
Width
Offset
Bolt pattern
Backspacing

As I’ve already mentioned, the key to getting the best rims for your car is by matching it to tire diameter and width.

Also, if you want to see how to measure your rims precisely, you can check out this article. It will show you what you need to measure them, how to refurbish them, and when to replace them.

Conclusion

The first time you stare at that long sequence on your tires, deciphering and reading it can seem like a daunting task. But as you can see, once you break down all of the letters and numbers, learning how to read rim and tire sizes is a piece of cake. Also, as soon as you get the hang of these, you’ll be able to pick out the best possible rims and tires for your car.

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By Car Roar