Once you have determined it’s time to buy tires, you’ll need to know what size tires are correct for your vehicle. Depending on what you drive, you may be interested in how to find the right tire for your car.
This information is usually inside your car’s doorjamb, in your owner’s manual. To ensure your current tire or a replacement tire you may be looking at matches your vehicle’s requirements, it will be good for you to understand how tire sizing works. You may have never paid attention to the string of numbers and letters on every tire, but it’s a gold mine of information.
How to Read a Tire Size?
Whether you’re switching out your tires for a different look or added performance, a good place to start your search for that new tread is the sidewall on one of your existing tires.
In addition to your tire’s sidewall, you may also find your tire size in the following places within your vehicle:
- Your driver’s side door jamb.
- Inside your glove box door.
- Within your gas tank hatch.
- In your vehicle’s owner’s manual.
The numbers and letters on that sidewall represent everything from the tire type and width to the aspect ratio, construction type, diameter, load index, and speed rating.
Here’s how to read the sidewall on your tires.
#1. Tire Type.
The letter “P” at the beginning of the “Tire Size” tells us the tire is a P-Metric tire, referring to tires made to certain standards within the United States, intended for Passenger vehicles.
If a tire size has no letters at the beginning, this indicates a Euro metric tire. P-Metric and Euro-Metric tires may have different load capacities.
The letters “LT,” either at the beginning or at the end of the tire size indicate the tire was designed for light trucks. Vehicle manufacturers equip some light trucks with “LT” type tires. These tires generally require higher inflation pressures than passenger tires.
Consult your owner’s manual or tire placard for the recommended tire size and inflation pressure for your vehicle.
#2. Width.
The first number to appear in your tire size information is the width, in millimeters, of the correct tires for your vehicle: P225/40R18 94Y.
Tire width always refers to the measurement from one sidewall to another. Thus, a tire with the measurement “P225” is for a passenger vehicle with a nominal width of 225 millimeters.
#3. Aspect Ratio.
After the slash mark, the next number you see is for the tire’s aspect ratio, which essentially tells you how tall your tire’s profile is: P225/40R18 94Y. Aspect ratios are delivered in percentages. Tire makers calculate the aspect ratio by dividing a tire’s height off the rim by its width. If a tire has an aspect ratio of 40, it means the tire’s height is 40% of its width.
Lower aspect ratio tires, such as a 60 series, generally offer vehicle handling performance advantages over higher aspect ratio tires, such as a 75 series, but a typical trade off can be ride harshness.
#4. Construction Type.
After the aspect ratio comes a letter that indicates the type of internal construction maintaining your tire’s stability: P225/40R18 94Y.
This single letter tells you about the internal construction of the tire. These include radial and bias.
R: Radial Tires
Tires with a radial construction will have the letter “R” in their tire size after the aspect ratio. Radial tires feature plies that are placed at 90-degree angles (radially) from one to the other sidewall.
This tire type is most common on modern roads, for a reason. The radial ply placement optimizes the tires for everyday use, as it minimizes the possibility of heat build-up.
D: Diagonal Tires
Diagonal tires, on the other hand, feature the letter “D” before the wheel diameter. Tires with this construction are made with nylon cord casing layers that are placed diagonally (at 55 degrees) in the tread and sidewall.
Often, diagonal models belong to the trailer category but other tire types are also manufactured with this construction. Passenger car and SUV tires are not available with diagonal constructions, as the primary function of these tires is their load durability.
Bias-Ply Tires
Technically, bias-ply tires are not marked with a letter. Yet, we need to mention them.
Tires with this construction type simply have a hyphen (-) between the aspect ratio and the rim measurements. This indicates that the tire’s internal ply cords are diagonally placed, overlapping each other. They are angled at 30-40 degrees in order to reinforce the tire’s strength.
#5. Rim Diameter.
The next number is the diameter code, in inches, of the rim onto which the tire can be mounted. For example, a tire with the P225/40R18 94Y would fit a rim with an 18-inch diameter.
#6. Load Index.
The two-digit or three-digit number that follows the gap specifies tire load index.
The load index symbol indicates how much weight a tire can support, based on the following standard chart (see below). In our example, the load index is 94. This means the tire has a load capacity of 1,477 pounds when inflated to the tire’s maximum air pressure rating.
Please note: Some run-flat tires are identified with an F followed by the type of internal construction.
| Load Index | Load (lbs) | Load Index | Load (lbs) | Load Index | Load (lbs) |
| 75 | 852 | 91 | 1356 | 107 | 2149 |
| 76 | 882 | 92 | 1389 | 108 | 2205 |
| 77 | 908 | 93 | 1433 | 109 | 2271 |
| 78 | 937 | 94 | 1477 | 110 | 2337 |
| 79 | 963 | 95 | 1521 | 111 | 2403 |
| 80 | 992 | 96 | 1565 | 112 | 2469 |
| 81 | 1019 | 97 | 1609 | 113 | 2535 |
| 82 | 1047 | 98 | 1653 | 114 | 2601 |
| 83 | 1074 | 99 | 1709 | 115 | 2679 |
| 84 | 1102 | 100 | 1764 | 116 | 2756 |
| 85 | 1135 | 101 | 1819 | 117 | 2833 |
| 86 | 1168 | 102 | 1874 | 118 | 2910 |
| 87 | 1201 | 103 | 1929 | 119 | 2998 |
| 88 | 1235 | 104 | 1984 | 120 | 3086 |
| 89 | 1279 | 105 | 2039 | 121 | 3197 |
| 90 | 1323 | 106 | 2094 | 122 | 3307 |
#7. Speed Rating.
The final figure in a tire size sequence is the speed rating indicated by a letter: P225/40R18 94Y. Just as your load index number corresponds to a particular load, your speed rating letter corresponds to a specific speed capability based on a standardized laboratory test.
For example, a tire with a speed rating “Y” is rated for up to 186 mph, while a tire rated “R” is up to 106 mph. Remember that this isn’t a recommended cruising speed, but rather the maximum speed that tire type can withstand. Of course, you should always follow legal speed limits on roadways.
Here are the most common letters for speed ratings and their corresponding maximum speeds:
- Q: 99 mph
- R: 106 mph
- S: 112 mph
- T: 118 mph
- U: 124 mph
- H: 130 mph
- V: 149 mph
- W: 168 mph
- Y: 186 mph
- ZR: 186+ mph
Tires with higher speed ratings tend to offer increased handling performance. Replacement tires must have the same or higher speed rating to maintain vehicle speed capability. If a vehicle has tires with different speed ratings, the speed rating of the “slowest” tire dictates the vehicle’s top speed.
Like the load index, the speed rating is not required by law to be listed on all tires. If your tires do not have the speed rating listed, your owner’s manual will have the relevant information for your model’s stock tire type.
How to Read the DOT Tire Identification Number?
The tire date coding, or DOT, shows when the tire was manufactured. While the DOT number is a longer number, which provides more information about the tire, its date code portion is the important part.
This is a four-digit number that tells us the week and year of the tire’s manufacture. For example, if the tire’s sidewall states 1020, that means the tire was in production on the 10th week of 2020.
Ply Construction
While tires no longer actually feature the number of plies their sidewalls state, the ply rating is still a crucial measurement. Therefore, “How to tell the ply on tires?” is our next section.
Tire brands display the ply rating and load range on the tire’s sidewall. They indicate the load durability of light truck tires. The ply rating shows the ply durability of the tire, in other words, how strong these tires are.
The ply rating is a number, which we count in even numbers starting from 10 for truck tires. However, tire manufacturers also mark passenger tires as well, and they all are 4-ply tires.
On the other hand, the load range is shown with letters, starting from A and going up. The higher this number is, the more load a light truck tire can handle.