Installing the correct size tires on your vehicle is critical to its performance. A common issue is that not everyone knows how to read the tire size. With many numbers and letters listed on the tire sidewall, it can be hard to understand what they mean.
This article will explain all of the tire sidewall specifications so that you will understand what the tire size means and all of the information.
Reading Tire Size
The three numbers you find on the sidewall of the tire are the width, height, and diameter of the tire. The width is measured in millimeters and is the distance from the edge of one side to the other side of the tire.
The height is specified in the height-to-width ratio. The diameter is specified in inches, which is the measurement of the rim or wheel that the tire fits onto.
For example, a simple reading of the tire would be a “225/50/17” tire with a width of 225, height of 50, and diameter of 17.
Tire sidewalls contain the following specifications, tire type, tire width, aspect ratio, and construction. They also typically have wheel diameter, load index, speed rating, DOT symbol, tire identification number (TIN), and UTQG. When using this information together, you can determine if the tire recommended is the correct size for your vehicle.
1. Tire Type.
P225/65R15 95H
A majority of car tires show the letter “P” at the front of the tire size. This letter indicates it is a P-Metric tire and is manufactured to the standards set forth for passenger vehicles.
If you see the letters “LT” instead, it means Light Trucks. Tire manufacturers make sure these tires have the higher inflation needs that some trucks require.
There are also other types in use, such as “T” for Temporary, “ST” for Trailer, and “C” for Commercial. However, there can also be chances that there is no letter on the tire. If this is the case, then it could be made in Europe. Euro-metric tires are comparable to our Passenger tires.
2. Tire Width.
P225/65R15 95H
The tire’s width can be measured. This measurement is in millimeters, so it is based on how much space exists between the two sidewalls. This measurement can also be known as the section width.
The first three numbers in the tire size are indicative of the tire’s width. So, with tire size P225/65R17, the width would be 225 millimeters.
3. Aspect Ratio.
P225/65R15 95H
After the slash mark comes the next number- the aspect ratio. The aspect ratio of a tire is the height from the cross-section to the width. This two-digit number is located after the slash symbol, as shown on the tire size.
In the example used above, P225/65R17, the 65 is the aspect ratio. The aspect ratio shows that the height of the tire is equal to 65% of the tire’s width. This number determines how the vehicle responds while handling the tires.
The greater the number then the longer the time in response relating to turning; and the shorter the number, shows quicker the response to a command.
4. Construction.
P225/65R15 95H
Most passenger tires feature an “R” after the aspect ratio. This letter is used to signify that it is a Radial Tire.
These tires are the most common tires on the road. They are constructed by assembling piles of fabric, steel, and polyester with a coated rubber that lies perpendicular to the direction that the tire travels. This design supports a smoother ride and longer tread life.
There can also be tires with a “D,” which is Diagonal Piles. This is generally called Bias-Cross Section, Bias Ply or X-Ply, Cross-Ply, and Conventional Tires; the cross-ply tire you will most often find is in the trailer or motorcycle tire.
If any tire does have an “F” before the construction letter that precedes the construction letter, which is generally only found in run-flat varieties.
5. Wheel Diameter.
P225/65R15 95H
It is essential to know what size wheel the tire is intended to fit. The last number in a tire size is, in fact, the wheel/rim diameter. This number indicates the appropriate size wheel for the tire.
The measurement is determined by measuring the distance across the center on a straight line from one end of the diameter to the other. For most tires, this number is listed in inches, such as 15 inches from our example, P225/65R15.
6. Load Index.
P225/65R15 95H
The load index is the number you use to indicate the overall weight the tire can carry when fully inflated. The higher the number in a tire’s load index rating, the more weight is able to be placed on the tires.
However, you are not just going to be able to read this number and instantly know how much weight it is capable of carrying.
You will have to reference another chart that tells you what the load index rating is. For example, if you have a load rating of 117, that means that tire can carry 2,833 lbs, which is readily available on a load index chart page. Most consumer vehicles sit between 70 and 125, which is a load index rating.
7. Speed Rating.
P225/65R15 95H
The speed rating or maximum speed rating is another number that you will need to reference with a chart. This number corresponds to the highest speed capability you can use the tire at. Tires with high speed ratings give you more control because you are able to drive at faster speeds and still manage heat.
Most family sedans, SUVs, and vans tires have S and T speed ratings, or roughly 112 or 118 mph. H is more common with passenger cars, SUVs, light pickup trucks, coupes, and sports cars that can reach 130 mph as a maximum speed. Other light trucks, vehicles used for recreation and service, etc., use N, P, Q, and R speed ratings (87, 93, 99, and 106 mph).
If you drive a high-performance vehicle, you may see a V, W, or Y ( 149, 168, or 186 mph). Almost all of the winter tires that are currently available will use Q, S, or T. Of course, this is just the tire’s maximum speed rating, and of course, you should always stay within the legal speed rating limits.
8. DOT Symbol.
Every tire sidewall that is legally used in the United States has the DOT symbol, which is the Department of Transportation, on the sidewall.
This indicator tells you that this tire meets the Federal Motor Vehicle Safety Standards. Any reputable manufacturer will have the DOT symbol on their tire, so rest easy when purchasing a major brand.
9. TIN.
Similar to how your car has an identification number, your tire also has a serial number! The Tire Identification Number (TIN) is a code assigned to every tire, with the only letters being the DOT code.
The TIN can be 8 through 13 letters/numbers to identify the most important characteristics of the tire. It tells you where the tire was manufactured, the manufacturer’s code, and the date the tire was manufactured.
10. UTQG.
The UTQG means Uniform Tire Quality Grading. It is a rating system created by the Department of Transportation. It gives you the information needed to purchase the safest tire based on temperature capability, traction, and treadwear.
The traction grade measures the wet traction ability of the tire as done in controlled testing. An AA-rated tire will give you the best traction when driving on wet roads.
The temperature grade measures the tire’s ability to properly handle and dissipate heat. The higher the temperature grade, the better it is at handling higher speeds. An A-rated A tire can handle speeds over 115 mph.
The treadwear grade measures how the tire performed under government testing. It is meant to show how much wear the tire’s tread should experience. The government test course is 6,000 miles long, long enough to be able to see how the tire should wear.
A tire graded at 150 would wear 1.5 times better than that same style of tire at 100 as the same tire. Consequently, a tire rated at 200 would last twice as long as that same tire rated at 100.
Still, how long a tire wears will depend on so much more than this grade. How the vehicle is driven (defensive or aggressive), the type of driving (urban vs. highway), and multiple other factors. The grading only signifies what the beating could take.
Is Tire Size Important?
The tires must fit your vehicle correctly for proper usage. The first problem with improperly sized tires is simply the way they look. When you put tires on that are too small or too large for your vehicle, it doesn’t look right.
But there are far more dangerous consequences of the wrong tire size. The tires carry the weight of your car and are the only part touching the road.
You should verify that the tires are rated for your particular vehicle for your on-road safety, or else you risk compromising your safety.
With the wrong tire size, you will feel compromised handling. If you try to turn or hit slippery roads, you might feel a change in your vehicle as well. Finally, tires that are not suited for your vehicle will wear faster, and you will spend money replacing them sooner.
Your car owner’s manual will recommend the tire size for your vehicle. You can also find this information on a placard in the driver’s side door jamb.
Tire Size Examples:
We will look at two popular types of tires and show you how to read them based on the factors above. As we look at these examples, you should be able to determine your own tire sizes using this same formula.
The first example is P225/65R17 100 T. If your tire says that, you would evaluate it based on the following factors:
- Tire Type: In this case, since it has a P, you know it is a passenger tire. This is the most common type of tire on the road.
- Tire Width: The width of the tire is 225 millimeters. This is measured sidewall to sidewall and is expected to be slightly above ten inches. If referred to as a section width.
- Aspect Ratio: The sidewall of the tire is 65% of the width of the tire. Based on that, the sidewall is about 146 millimeters or about 5.75 inches.
- Construction: The R stands for Radial Tire. This is also the most common type of tire on the road.
- Wheel Diameter: The diameter is 17 inches. This is measured edge to edge and tells what size wheel to put the tire on.
- Load Index: If the load index of the tire is rated at 100, the tire can carry up to 1,604 pounds.
- Speed Rating: This tire also has a speed rating of T. This tire is good for speeds up to 118 mph.
In review, this P225/65R17 100T tire is a passenger tire that has a width of 225 millimeters. It has an aspect ratio of 65% and a radial construction. It has a 17-inch diameter, can carry 1,604 pounds, and travel at speeds up to 118 mph.
Let’s go through one more example to make sure that you understand. For this example, we will use P275/55R20 111S.
- Tire Type: In this case, since it has a P, you know it is a passenger tire. This is the most common type of tire on the road.
- Tire Width: The width of the tire is 275 millimeters. This is measured sidewall to sidewall and is expected to be slightly shorter than 11 inches.
- Aspect Ratio: The sidewall of the tire is 55% of the width of the tire. Based on that, the sidewall is about 151 millimeters or about 6 inches.
- Construction: The R stands for Radial Tire. This is also the most common type of tire on the road.
- Wheel Diameter: The diameter is 20 inches. This is measured edge to edge and tells what size wheel to put the tire on.
- Load Index: If the load index of the tire is rated at 111, the tire can carry up to 2,403 pounds.
- Speed Rating: This tire also has a speed rating of S. This tire is good for speeds up to 112 mph.
In review, this P275/55R20 111S tire is a passenger tire that has a width of 275 millimeters. It has an aspect ratio of 55% and a radial construction. It has a 20-inch diameter, can carry 2,403 pounds, and travel at speeds up to 112 mph.
Where to Find The Tire Size?
When you are getting new tires on your vehicle, you want to make sure you are getting the right tire size. There are two ways to reference the tire size of your vehicle.
One, look at the tires on the vehicle now. One way to match this information is to have the same tires put on your vehicle. But what happens when you discover that someone put the wrong tires on your vehicle before you purchased it.
That is why it is always recommended to go to the door jamb inside the driver’s door. There is a placard that will tell you the correct tire size. Another place to find this information is in your owner’s manual. When you check with the car manufacturer’s specifications, you can’t miss.
What do the 3 numbers mean on tire size?
The 3 numbers on tire size mean width, height, and diameter, in that order. The width is the tire width in millimeters and measures how wide your tire is. The height is calculated in a height-to-width ratio. The diameter is in inches and determines the size of the rim or wheel that the tire will fit on.
For example, if you have a tire that is width of 225, a height of 50, and a diameter of 17 – it is a 225/50/17 tire.
How to read tire size on a truck?
Most trucks use the same tire size system as cars do. The first number in a truck tire size is the width of the tire in millimeters.
The second number is the height or aspect ratio of the tire. The third number is the diameter of the wheel in inches. In the next section, we provide information on truck tire sizes in inches.
How to read tire sizes in inches?
Tire sizes in inches can be a little more complicated than tire sizes in millimeters. For example (35X12 50R18) – The first number of 35 and 12 refer to the diameter and the width of the tire, respectively, while the number 50 would refer to the aspect ratio (the height of the sidewall compared to the width).
The last R18 refers to the rim in inches. So, for this example, a 35X12 50R18 would be a 35-inch tire and would have a width of 12 inches and would fit on an 18-inch rim.
How to read a motorcycle tire size?
Motorcycle tires use the same characteristics for sizes as car tires. The first 3 digits are the width of the tire in millimeters (mm).
The next two digits are the aspect ratio, which refers to height when measured from its cross section to its width. Last, the final two digits describe the diameter of the motorcycle wheel, in inches.
Tire Size Charts
| Load Index Chart | Speed Rating Chart | |||||
| Load Index | Load (lbs) | Load Index | Load (lbs) | Symbol Speed | Speed (km/h) | Speed (mph) |
| 71 | 761 | 100 | 1764 | A1 | 5 | 3 |
| 72 | 783 | 101 | 1819 | A2 | 10 | 6 |
| 73 | 805 | 102 | 1874 | A3 | 15 | 9 |
| 74 | 827 | 103 | 1929 | A4 | 20 | 12 |
| 75 | 852 | 104 | 1984 | A5 | 25 | 16 |
| 76 | 882 | 105 | 2039 | A6 | 30 | 19 |
| 77 | 908 | 106 | 2094 | A8 | 40 | 25 |
| 78 | 937 | 107 | 2149 | B | 50 | 31 |
| 79 | 963 | 108 | 2205 | C | 60 | 37 |
| 80 | 992 | 109 | 2271 | D | 65 | 40 |
| 81 | 1019 | 110 | 2337 | E | 70 | 43 |
| 82 | 1047 | 111 | 2403 | F | 80 | 50 |
| 83 | 1074 | 112 | 2469 | G | 90 | 56 |
| 84 | 1102 | 113 | 2535 | J | 100 | 62 |
| 85 | 1135 | 114 | 2601 | K | 110 | 68 |
| 86 | 1168 | 115 | 2679 | L | 120 | 75 |
| 87 | 1201 | 116 | 2756 | M | 130 | 81 |
| 88 | 1235 | 117 | 2833 | N | 140 | 87 |
| 89 | 1279 | 118 | 2910 | P | 150 | 94 |
| 90 | 1323 | 119 | 2998 | Q | 160 | 100 |
| 91 | 1356 | 120 | 3086 | R | 170 | 106 |
| 92 | 1389 | 121 | 3197 | S | 180 | 112 |
| 93 | 1433 | 122 | 3307 | T | 190 | 118 |
| 94 | 1477 | 123 | 3417 | U | 200 | 124 |
| 95 | 1521 | 124 | 3527 | H | 210 | 130 |
| 96 | 1565 | 125 | 3638 | V | 240 | 149 |
| 97 | 1609 | 126 | 3748 | W | 270 | 168 |
| 98 | 1653 | 127 | 3858 | Y | 300 | 186 |
| 99 | 1709 | 128 | 3968 | |||
Now that you know how to read your tire size, you will be ready the next time you are in the market for new tires. There is a lot of information on the sidewall of a tire, but it is much less complicated than it looks.
Yet, there are many other forms of tire sizes, many of which can be represented in different ways. These, in order of most common to least common, are representations of tire sizes:
- Metric Tire Size
- Alpha-Numeric Tire Size
- P-Metric Tire Size
- LT-Metric Tire Size
- Numeric Tire Size
Metric Tire Size
The metric tire designation uses a three-digit number to present the tire’s width in millimeters, a two-digit aspect ratio to show the tire’s height as a percentage of the tire’s width, and a letter indicating the type of construction (for example, R = radial). This is the most common method of sizing tires.
Alpha-Numeric Tire Size
This method of tire sizing is an older way to indicate the width, aspect ratio, and rim diameter of the tire, using a combination of letters and numbers.
P-Metric Tire Size
The P-Metric method of tire sizing uses a similar method of abbreviation for designation that is similar to the metric method, but with the addition of a “P” at the front of the tire size. A “P” denotes that the tire is specified for use on passenger cars.
LT-Metric Tire Size
The “LT” at the front of the tire size indicates that the tire is specified for use on light trucks. This is the only difference from the P-Metric method of tire sizing.
Numeric Tire Size
The numeric method for tire sizing is an older method that uses a two-digit number to specify the width of the tire in inches, another two-digit number to indicate the aspect ratio, and finally, a letter to identify the type of construction of the tire.
So, once you know what to look for, reading your tire size is easy. So, by knowing how to read your tire size, and now knowing what the numbers mean on the tires, you can now buy the perfect tires yourself and make sure you have the best tires on your vehicle.