Home / Understanding Metric Tire Measurements
Understanding Metric Tire Measurements
| Metric to Imperial tire conversion chart | |||||||
| Metric = | Imperial | Metric = | Imperial | Metric = | Imperial | ||
| 215/75/15 | 27.7"x 8.5" | 245/75/16 | 30.5"x 9.6" | 315/70/17 | 34. |
||
| 225/70/15 | 27.4"x 8.9" | 255/70/16 | 30.0"x 10.0" | 275/65/18 | 32.1"x 11.0" | ||
| 225/75/15 | 28.3"x 8.9" | 265/70/16 | 30.6"x 10.4" | 375/50/18 | 33.0"x 15.4" | ||
| 235/75/15 | 29.0"x 9.3" | 265/75/16 | 31.6"x 10.4" | 325/60/18 | 33.2"x 13.3" | ||
| 245/75/15 | 29.5"x 9.6" | 275/70/16 | 31.2"x 10.8" | 325/65/18 | 34.8"x 13.5" |
||
| 255/75/15 | 30.0"x 10.0" | 285/75/16 | 32. 8"x 11.2" |
305/70/18 | 35.0"x 12.2" | ||
| 265/75/15 | 30.6"x 10.4" | 295/75/16 | 33.4"x 11.6" | 375/60/18 | 35.7"x 15.5" | ||
| 205/85/16 | 29.7"x 8.1" | 305/70/16 | 33.0"x 12.2" | 395/65/18 | 37.8"x 15.7" | ||
| 215/75/16 | 28.7"x 8.5" | 315/75/16 | 34.8"x 12.9" |
325/50/20 | 33.0"x 13.2" | ||
| 225/70/16 | 28.4"x 8.9" | 375/55/16 | 32.8"x 15.5" | ||||
| 225/75/16 | 29.2"x 8.9" | 395/70/16 | 37.
9"x 16.0" |
||||
| 235/70/16 | 29.0"x 9.3" | 245/70/17 | 30.6"x 9.8" | ||||
| 235/85/16 | 31.7"x 9.3" | 265/70/17 | 31.6"x 10.7" | ||||
| 245/70/16 | 29.5"x 9.6" | 285/70/17 | 33.0"x 11.5" |
||||
Example: 225/70R15
| 225 | = Section width in millimeters |
| 70 | = Aspect ratio (ratio of section height to section width) |
| 15 | = Rim diameter |
| R | = Radial Construction B = Bias Construction D = Bias Belted Construction |
To calculate:
| Tire Width: | Section width divided by 25. 4 (Ex: 225 / 25.4 = 8.86") |
| Section Height: | Tire width times aspect ratio (Ex: 8.86" x 70% = 6.20") |
| Overall Diameter: | Section height times 2 plus rim diameter (Ex: (6.20" x 2) + 15 = 27.40") |
Home > Company > Tire Safety > Choosing Tires > Determining Tire Size
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…
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.
If you’re unsure of how to read tire measurements from your tire walls, the information and graphics below will tell you how to read tire size, understand and interpret it. If you decide you want to substitute a new size or tire type, consult an authorized tire retailer who can expertly advise you, because many optional tire sizes may have different load capacities and could require wheels of a different rim width or diameter and different inflation pressure.
Not sure you need new tires? Our Tire Replacement Guidance article will help you determine whether it’s time to retire your tires.
Most passenger cars, SUVs and light pickups (1/2 ton and smaller) will come with tires that are either P-Metric or Euro-Metric.
For P-Metric tires, you’ll see the letter “P” before the number sequence begins: P225/70R16 97H. P-metric is a designation standardized by the Tire and Rim Association for a “passenger car” tire type. For Euro-Metric there will be no preceding letter before the number sequence begins: 225/70R16 98H. Euro-Metric is a designation standardized by the European Tyre and Rim Technical Organization for a “passenger car” tire type. Both P-Metric and Euro-Metric size tires are designed to primarily be used on passenger vehicles, which can include cars, minivans, SUVs, and other light duty pickup trucks.
If your vehicle is an SUV, Pickup truck or van, you might see a different type of size designation on your placard that is specific for heavy duty light trucks and vans, especially common on ¾ ton and larger pickup trucks and vans. There are two common size types in this category, LT-Metric and Euro-Metric Commercial (aka C-type). Both size types are metric and so use the same structure as P-Metric and Euro-Metric but have some different characters in the size that differentiate them from their passenger car cousins.
LT-Metric tires will have the letters “LT” before the size number sequence: LT245/75R17 119/116R Load Range E. Notice that there are two load index numbers and a Load Range, see the section on Load Index for more info. LT-Metric is a designation standardized by the Tire and Rim Association for a “light truck” type tire. Euro-Metric Commercial or C-Type tires will look very similar to a passenger Euro-Metric size except that there will be a “C” right after the rim size: 23/65R16C 121/119R. Notice that the C-type tires also have two load index numbers. Euro-Metric Commercial, or C-Type is a designation standardized by the European Tyre and Rim Technical Organization for a light truck type tire. Light truck tires are designed to be used on vehicles capable of carrying heavy cargo and are usually only specified by a vehicle manufacturer on vehicles exceeding a certain load capacity.
Other types of tires that fall into the Metric sizing type are Temporary Spares, they start with “T”.
If you see a size that starts with “ST,” that means “special trailer” and is only for use on a trailer.
Regardless of whether you are looking at a P-Metric, Euro-Metric, LT-Metric, Euro-Metric Commercial, T or ST tire the numbers in the size mean the same thing.
The first number to appear in your tire size information is the width, in millimeters, of the correct tires for your vehicle: P225/70R16 91S.
Tire width always refers to the measurement from one sidewall to another. Thus, a tire with the measurement “P225” is for a passenger vehicle and has a nominal width of 225 millimeters.
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: P225/70R16 91S. 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 70, it means the tire’s height is 70% 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.
After the aspect ratio comes a letter that indicates the type of internal construction maintaining your tire’s stability: P225/70R16 91S.
There are two types of construction that you may see on the sidewall of a tire:
Radial tires are the most common tires on the road in the United States today; thus “R” will usually be shown in the tire size designation. Radial construction means the tire’s internal ply cords are oriented in a radial direction, from one bead over to the other, essentially perpendicular to the direction of rotation.
You may also occasionally see RF indicating a run flat tire or ZR indicating a tire that is a speed rating higher than V.
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 P225/70R16 91S would fit a rim with a 16-inch diameter.
Load index can be a confusing subject because there are so many different caveats, but we will try to explain everything here.
The next figure after the rim size in the sequence is your tire’s load index, which tells us how much weight, in pounds, the tire can support when fully inflated: P225/70R16 91S
We call it the load “index” because the number doesn’t tell us the precise number of pounds the tire can carry, at least not by itself. However, the number does correspond to a specific load capacity listed in an index.
Beginning with 1 and ending with 150, numbers in the load index represent carrying capacities of 99 to 7385 lbs.
There are two types of load types for passenger tires though, Standard Load and Extra Load. If a tire is Standard Load there will be no markings indicating it but if it is Extra Load the letters XL will appear after the size and load index.
Standard Load Euro-Metric: 215/55R17 94V
Extra Load Euro-Metric: 215/55R17 98V XL
Passenger car tires like P-Metric and Euro-Metric will only have one load index number where LT-Metric and Euro-Metric Commercial (C-Type) will have two numbers separated by a slash. The first number is the load index if the tire is used in a single application, the second number is the load index if the tire is used in a dual application. Passenger type tires cannot be used in a dual application. Light truck tires will also have a Load Range that is indicated by a letter, such as Load Range E. Load Range is an older term that is still commonly used in the industry so you may hear your tire dealer reference it but the load index numbers are the best way to ensure you have the proper tire.
One important but often misunderstood facet about load index is that the load index numbers between standards organizations (P-Metric vs Euro-Metric) are not necessarily on the same scale. Meaning that two tires in the two different systems that have the same load index number could have different maximum load capacities. This is why it’s important to not only look at the load index number but also verify the actual load capacity.
The final figure in a tire size sequence is the speed rating, which is indicated by a letter: P225/70R16 91S. Just as your load index number corresponds to a specific load, your speed rating letter corresponds to a particular speed capability based on a standardized laboratory test.
For example, a tire with speed rating “S” is rated for up to 112 mph, while a tire rated “R” is up to 106 mph. Remember that this isn’t a recommended cruising speed. Of course, you should always follow legal speed limits on roadways.
Replacement tires must have the same or higher speed rating as the vehicle’s Original Equipment to maintain vehicle speed capability. If a vehicle has tires with different speed ratings, it is the speed rating of the “slowest” tire that dictates the vehicle top speed.
There is one last sizing type that you should know about, especially if you are in the market for off road tires for a light truck or SUV. It’s called a Flotation size and the numbers in this sizing format are very different from the Metric formats. Flotation sized tires are similar to LT-Metric tires in application except for a few important points. Number one, they cannot be used in dual applications and number two, an equivalent size tire may have different load capacity than its LT-Metric counterpart.
The first number in the Flotation tire size is the overall diameter in inches.
Pretty straight forward.
The second number is the section width (sidewall to sidewall) measurement in inches. Again, fairly simple.
After the section width comes a letter that indicates the type of internal construction: 33X12.50R17LT 120Q.
This is the same as is found in the metric sizing systems.
There are two types of construction that you may see on the sidewall of a tire:
Radial tires are the most common tires on the road in the United States today; thus “R” will usually be shown in the tire size designation. Radial construction means the tire’s internal ply cords are oriented in a radial direction, from one bead over to the other, essentially perpendicular to the direction of rotation.
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 33X12.50R17LT 120Q would fit a rim with a 17-inch diameter.
The letters LT will be after the Rim Diameter indicating that this tire type is intended for Light Truck vehicles similar to the LT-Metric and Euro-Metric Commercial (C-Type) tires.
Load Index and Speed Rating have the same meaning and format as the tires using the metric sizing system. Note that since flotation tires cannot be used in a dual application there will be only one load index number instead of two.
Another group of stamping on certain types of tires is the Uniform Tire Quality Grading or UTQG. This grading and stamping is required for passenger car tires (i.e. P-metric and Euro-metric) in the all season and summer categories.
Dedicated winter tires, Light Truck (LT-Metric, Euro-Metric Commercial, Flotation) and Motorcycle tires are excluded from this requirement.
Quality grading is designed to make the tire purchase decision easier for you. Ideally, the system is intended to provide simple, comparative data so you can make an intelligent buying decision. However, the ratings are based upon test results achieved under special conditions. This means it’s possible to misinterpret the comparative data as it relates to your individual driving habits, conditions, etc. You should still rely on your service or tire professional for assistance.
Quality grading designates the comparative performance levels of a tire based on government-specified tests but commissioned by the individual tire manufacturers. All tire manufacturers are required to grade regular and all-season passenger tires in three categories:
UTQG
Treadwear
The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test course for 6,000 miles (9,600 km).
For example, a tire graded 150 would wear one and a half times as well on the government course as a tire graded 100. However actual tire performance depends on driving habits, road characteristics, service practices, and other factors that can influence the outcome.
Traction Grades AA, A, B and C
The traction grades from highest to lowest are AA (the highest), A, B and C. They represent how well tires stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. C-rated tires will have the lowest traction performance.
WARNING: THE TRACTION GRADE ASSIGNED IS BASED ON A WET BRAKING (STRAIGHT AHEAD) TRACTION TEST AND DOES NOT INCLUDE CORNERING (TURNING) TRACTION.
Temperature Grades A, B and C
The temperature grades A, B, and C represent the tire's resistance to the generation of heat and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel.
Sustained high temperature can cause the tire’s material to degenerate and reduce tire life, and excessive temperature can lead to sudden tire failure. The grade C corresponds to a performance level all passenger car tires must meet under the Federal Motor Vehicle Safety Standard No. 109. Grades A and B represent higher levels of performance on the laboratory test wheel than the minimum required by law.
WARNING: THE TEMPERATURE GRADE IS ESTABLISHED FOR A TIRE THAT IS PROPERLY INFLATED AND NOT OVERLOADED. EXCESSIVE SPEED, UNDER INFLATION, OR EXCESSIVE LOADING, EITHER SEPARATELY OR IN COMBINATION, CAN CAUSE HEAT BUILDUP AND POSSIBLE TIRE FAILURE.
DOT Quality Grades
All passenger car tires must conform to other federal requirements in addition to these grades.
Learn about our mission, our achievements, and our ongoing commitments
Read Our Origin Story
Tire size is the main parameter when assembling car wheels.
Among drivers, the dimensions of tires are called - standard size. Each disc is designed for a specific tire parameter. Mismatch of factory specifications, and the tires will not fit on the disk due to the small size, or vice versa - they will be too large. What size it is and how it is indicated on the wheel, we will analyze in detail further.
All drivers paid attention to the large numbers displayed on the side of the wheel in the following order 000/00 R 00. The marking is applied in the indicated sequence. Values may vary. The affixed symbols characterize the size of the tire.
We will consider the example of tires with a size of 185/55 R 14:
Refer to your vehicle owner's manual to find out what size tire your car manufacturer recommends. In the technical specifications / wheels / section, the manufacturer indicates 2-3 types of tires that are recommended for use when operating the machine in different seasons.
In addition, it is permissible to refer to online or offline directories, where, by the make and model of the vehicle, it is possible to inquire about the size of tires. In addition, some machines are equipped with tire size and recommended pressure plates. The Japanese prefer to inform the car owner in this way.
The performance and maneuverability of the machine depend on the final choice of tires. Even if the tire is seated in the rim seat, this is not a sign of correct selection.
Let's analyze the influence of each tire parameter on driving performance:
The main question in disputes between car owners - how does the width of the profile affect performance? Someone argues that the wider the rubber, the better. Others, on the contrary, prefer narrow wheels. The truth is in the middle. In this case, it is worth considering the fact that for different operating conditions an optimal tire size is required.
As noted above, the width of the profile is directly related to the tread. The higher this value in mm, the larger the area of the contact patch. And this affects such characteristics as: directional stability, handling, fuel consumption, braking distance and the degree of aquaplaning.
If you operate a car with a volume of up to 2.0 liters in the city, then it is better to choose medium or narrow tires. The fact is that wide tires will be useless on city streets. And in the case of equipping a car with a powerful power plant and regular trips along the highway, it is recommended to choose tires with a wider profile, but listening to the recommendations of automakers.
The height of the tire profile has become the subject of tuning sports and youth cars in recent years. A lot of motorists are swapping factory rims for more attractive larger diameter wheels, completing the new thing with low profile tires. At the same time, many do not realize that in addition to the appearance, the technical characteristics of the car are changing: handling, maneuverability, fuel consumption, etc.
In addition, the height of the profile affects the rigidity and comfort of movement, suspension performance and protection of discs from damage. The low profile is good at speed and cornering. But its characteristics noticeably melt off-road, where soft and high rubber is more practical. It increases ground clearance, protects the discs and hard contact suspension. Therefore, when choosing the height of the tire profile, you should be guided by the conditions of the landscape.
The outer radius of the wheel, makes a noticeable change in the character of the car.
For example, by installing tires with a low profile, the car will accelerate faster. However, the distance traveled on the speedometer will be longer than operating the car on a high profile. The fact is that at the same crankshaft speed, the rotation of the wheels will be different, since the circumference of the tires can have a different distance, and as a result, an unequal area (2 * pi * R). That is, for one complete revolution of the wheel, the distance traveled will be different. The smaller the radius of the wheel, the greater the mileage.
Follow the recommendations of the car manufacturer and complete the wheels only with the rubber that is indicated by size in the technical manuals and manuals. Experiments can significantly deplete the budget. In addition, experiments are dangerous not only for the car owner, but also for others.
How to choose the right tires for your car? In the off-season, for many drivers, this is the most pressing issue.
The necessary information about the sizes, speed and loading characteristics of tires is easy to find in the car's operating manual. Sergey Mishin, the head of the tire test group of the Za Rulem magazine, talks about how to navigate the tire markings and what threatens mistakes in choosing winter and summer tires.
Tire test group ZR answers questions
The main "metrics" of the tire are hidden in a set of numbers printed on the sidewall. Take, for example, 195/65R15.
The first (195) indicates the width of the tire profile in millimeters. The second (65), after the slash, is a series of tires. In fact, this is the height of its profile, indicated as a percentage of the width. In more familiar millimeters, it will be: 195 × 0.65 \u003d 126.75.
The letter R indicates the radial design of the tyre. And the last digit (15) indicates the landing diameter (not the radius!) of the tire in inches.
Converting it to millimeters (15 × 25.4 = 381) and adding twice the height of the profile (or sidewall) in the same units (126.75 × 2 = 253.5), we get the main overall size of the tire - its outer diameter: 381 +253.5 = 634.5 mm.
The owner's manual will tell you which tires are suitable for your car. The lazy ones will find a label with allowable dimensions and recommended pressure on the end of the driver's door or the B-pillar.
If the tire diameter is less than optimal, this will lead to a decrease in ground clearance, and the use of larger diameter tires is limited by the dimensions of the wheel arches. Rubber should not touch the body or chassis elements - especially when turning the wheels left and right and driving with a load on rough roads. Any deviation from the recommended diameter also affects the speedometer readings.
What tire labels say
Za Rulem.RF found out how readers relate to bald tires
If the car manufacturer allows you to vary the tire size within certain limits, please note that wider tires are preferable for summer: the wider the contact patch with the road, the better the tires cling to it.
But the wider the tire, the greater the rolling resistance. In addition, on such tires, the car is less well controlled in small radius turns - the tread has to slip due to the fact that its opposite sides pass different paths. Another disadvantage of wide tires is a tendency to hydroplaning (loss of contact with a wet road).
The high sidewall softens bumps well, but makes the tire more pliable. When turning, it deforms and does not respond so quickly to taxiing. Therefore, from the point of view of handling and stability, a low profile tire is preferable. But it is stiffer and has less durable sidewalls.
In winter, on the other hand, the specific pressure in the contact patch should be higher, which improves grip on ice and snow. So, narrower tires are preferable here.
In the marking, as a rule, there are a couple more numbers that are rarely paid attention to, for example, 91H, 95T. These are indexes of carrying capacity and maximum permissible speed.
According to special tables, it is not difficult to convert them into specific values of load (in kilograms) and speed (in kilometers per hour).
Here is an example calculation for a maximum speed of 185 km/h. We increase the figure by 15%, since such an increase is possible when driving on a long descent or with a strong tailwind. 185 x 1.15 = 209,3. According to table No. 1, this number is between 190 and 210. We round it up and get the index H.
). But in no case should you exceed the speed limit of tires - they will not withstand a large centrifugal force.
Now about the carrying capacity. The number 82, for example, indicates that the tire is capable of carrying a load of no more than 475 kg. Determine the minimum required for your vehicle based on the maximum axle load at full weight. Divide this value by two and in the tables (they are shown in the photo gallery at the bottom of the article) select the nearest value, rounding it up. Some tire and car manufacturers recommend increasing the calculated value by 20%, creating a margin.
On some tires, this set is supplemented by some more symbols.
Supertest 18 sets of winter tires: driving into winter
How drivers choose tires
XL is a pure bluff, designed to suggest that tires have increased load capacity. But the real load is determined only by the index described above.
SUV or 4×4 (depending on the manufacturer) indicate that the tires are designed for crossovers or all-terrain vehicles. Their main features are a more powerful frame and shoulders reinforced from the inside.
The letter C in the label (eg 185/75R16C) refers to light commercial vehicles (minivans and light trucks). Such tires are distinguished by a double index of load capacity (for example, 104/102Q), where the first digit indicates the load capacity of the wheel in a single-wheel version, and the second - in a two-wheel version.
Run-flat tires are able to travel some distance without losing air.
