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There are 3 common width measurements when talking about tires.
The measurements listed on a tire's sidewall are the product of a standardized system that is used by all tire manufacturers. A tire's section width is measured:
The width of the measuring rim is determined by the tire size. The assigned measuring rim width changes with the tire's Section Width and Aspect Ratio. The narrow measuring rims are assigned to narrow tires with taller sidewalls. The wider measuring rims are assigned to wider tires with lower profiles.
| Tire Size | Measuring Rim Width |
| P175/75R14 | 5.0 in. |
| P195/75R15 | 5.5 in. |
| P215/75/R15 | 6.0 in. |
| P235/75R15 | 6.5 in. |
| P225/75R15 | 6.
0 in. |
| P225/70R15 | 6.5 in. |
| P225/65R16 | 6.5 in. |
| P225/60R15 | 6.5 in. |
| P225/55R15 | 7.0 in. |
| P225/50R15 | 7.0 in. |
| P225/45R17 | 7.5 in. |
| P225/40R18 | 8.0 in. |
Manufacturers also designate a minimum and maximum wheel width that a tire can be safely mounted on. The width of the rim will affect the section width of the tire.
Answer ID 4652 | Published 06/21/2016 09:23 AM | Updated 03/06/2019 03:22 PM
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.
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1.
What is the tire profile height and how to calculate it?
2. What does the tire profile affect?
3. Increasing the tire profile: pros and cons
4. Reducing the tire profile: pros and cons
5. Do I need to change the profile height?
The tire profile is one of the key parameters that you need to pay attention to when choosing car tires. For each car, the manufacturer usually develops recommendations regarding what tire size should be and what deviations in these dimensions are acceptable. Deviations from the specified especially often concern the height of the tire profile. What affects the profile of the tire, and is it really necessary to change it? We understand.
What is tire profile height and how to calculate it?
Simply put, the height of a tire profile is practically its thickness, that is, the distance from the disk to the outermost surface of the tread.
The catch is that in the markings on the car tire itself, the profile height is not indicated directly in millimeters - it depends on the width of the tire.
For example, the marking says that we have a tire of dimension:
215/50 R17
The width is 215 mm, the profile height is 50% of this width, which means: 215 * 0.5 = 107.5 mm. What is important in this system of calculation? That with different tire widths with the same height percentage, the height will actually be different. So, for 225/50 tires, the profile height will already be 225 * 0.5 = 112.5 mm. So don't let the coincidence of the numbers after the fractional line on the marking mislead you - with different widths, the height will also be different.
Marking R17 in colloquial speech is often called the radius of the wheel, but this is not true. R stands for radial construction (how the cord fits in the tire), and the number is the fit or inner diameter of the tire in inches. So, we have 17-inch radial tires.
The labeling of American car tires is different from European ones. It is presented in the form
35×12.
50 R15
There is no profile height data here, and all dimensions are measured in inches. 35 inches is the outer diameter of the tire, 12.50 is the width of the tire, and R15 is the already familiar 15 inches of the landing diameter. To calculate the profile, we connect geometry to arithmetic: subtract the inner diameter from the outer diameter, we get the “thickness” of the tire on both sides of the disk, divide it by two and get the profile. (35-15)/2= 10 inches. You can convert to meters according to the scheme 1 inch = 2.54 cm. Thus, the tire profile is 25.4 cm or 254 mm.
What is the profile of a tire?
As practice shows, even a slight deviation from the tire parameters specified by the manufacturer can affect how the car behaves on the road. For example, tires with a low profile and larger rims are easier to handle in turns, and a high tire profile increases flotation. It turns out that even with small changes in the height of the profile, you can slightly adapt the car to your driving style and road conditions.
How far you can deviate from the optimal dimension is another question. It is possible to reduce and increase the profile of the tire without changing the diameters of the wheel; it is enough to choose a larger or smaller disk, respectively. If the entire diameter of the tire increases, then problems may arise: when driving, the tire will touch the wheel arch or fender liner, which threatens with damage. And the wheel itself will become noticeably heavier, which will put an additional load on the suspension. Such changes are well tolerated by SUVs, since they are designed for additional load, but owners of ordinary passenger cars should think again.
Increasing the tire profile: pros and cons
Pros:
- while maintaining the width of the tire, a slight increase in the profile will make driving softer, especially when passing obstacles;
- reduces the risk of tire sidewall puncture;
- tires with a larger diameter (by 1-2 sizes) give a higher fit, which increases the patency of the tire.
Minuses:
- too high a tire profile will have a bad effect on braking and cornering performance;
- with an increase in the diameter of the tire, there is a risk of touching the body parts with tires, especially when the steering wheel is fully turned;
- Tires with a high profile weigh more, which gives an additional load on the suspension and the tires themselves when driving.
Tire profile reduction: pros and cons
Reducing the tire profile is not so often resorted to. This is usually due to the desire to give the car a more stylish and aggressive look. Tires with a low profile and a wide rim are usually reserved for sports cars and high-end cars with a sporty slant. And not in vain.
Pros:
- low-profile tires improve the car's responsiveness to handling and braking performance;
- tires with a low profile are more resistant to deformation during maneuvers and cornering;
- better grip on flat roads.
Minuses:
- low-profile tires are demanding on the quality of the road, since driving comfort on such tires drops sharply on uneven bumpy surfaces;
- tires with a low profile are noisier;
- the risk of deformation during long-term parking due to high pressure and an increased contact patch of the tire with the surface.
Do I need to change the height of the profile? conclusions
1. Low-profile tires are adapted to high-speed driving on highways and flat highways; they are unlikely to show their best side on domestic roads.
2. Increasing the profile of the tire while maintaining the diameter of the disk increases the vehicle's cross-country ability; an increase in the profile with a corresponding reduction in the disc increases driving comfort on rough roads.
3. Manufacturers advise deviating from the recommended tire profile sizes by no more than 3% for passenger cars. For trucks and light trucks, the spread can be more flexible.
4. Changing the tire profile affects how your car behaves on the road. Responsiveness, softness of movement, speed of acceleration and braking, and many other characteristics will change, which will have to get used to again.
Tire parameters
Some motorists, when buying wheels for their car, may find it difficult to determine the profile of the tires that are installed on their car.
But in fact, everything is quite simple - the profile is indicated in the marking on the sidewall of the tire, you just need to read it correctly.
So the tire size 205/60/R15 means that the width of the tire is 205 mm, its profile (height) is 60% of the width (123 mm), and R15 means the type (radial) and the landing diameter in inches 15" = 381 mm
From this article you will learn:
To achieve better vehicle handling (and more impressive appearance), some motorists fit low profile tires with larger diameter rims to their car.
Typically used by motorists who prefer to drive fast and aggressively.
Low profile tires actually help to improve the car's handling - even without major intervention in the car's suspension, they provide fairly confident cornering at higher speeds than standard tires.
However, along with improved handling, low-profile tires make the car much more rigid and bumpy - all the bumps in the road will be felt by the driver and passengers in detail.
There are also quite a few drivers who prefer the opposite - to increase the tire profile - trying to achieve a higher seating position and slightly better car cross-country ability, which is important for Russian roads.
Slightly larger tires can actually lift the car off the road noticeably.
For example, the machine is equipped with tires 205/60/R15 with a profile (height) of 123 mm.
If the car owner wants to install tires of the following size - 205/65 / R15 - then these tires will raise his car by a whole centimeter - the profile (height) of these tires will be 133.
