Tire size can be confusing. Some numbers on the sidewall are listed in millimeters while others are inches. Plus, the right size for your car, truck, or trailer can differ depending on where and how you drive.
You can see your original equipment tire size in your owner’s manual or on the placard generally located on the driver’s side door jam. This is the sizing recommended by the vehicle manufacturer.
If you’re interested in switching out your tires for a different look or performance, a good place to start is the numbers and other indicators on your existing tires’ sidewall. Next, have a tire professional help you determine a tire size range that will fit your vehicle and driving needs.
Here’s what those numbers and indicators on the sidewall indicate and how to understand them:
A: TIRE TYPE The first letter in the code tells you what class of tire it is.
P stands for passenger vehicle tire. P-class tires include cars, SUVs, crossovers, minivans and smaller pickup trucks.
LT means light truck tire, designed for vehicles that are capable of carrying heavy loads, towing trailers, or for those looking for an extra heavy duty option. These are often equipped on three-quarter or 1 ton trucks and SUVs.
ST stands for Special Trailer. These tire sizes are meant for trailers, including fifth wheels and other travel trailers, as well as boat and utility trailers.
If there’s no letter before the first number, you have a metric tire most commonly referred to as European size. It’s also measured in millimeters but may have a different load capacity than a P or LT tire.
B: TIRE WIDTH The three-digit number following the letter is the tire’s width (from side to side, looking at the tire head on) in millimeters. This may also be referred to as the section width.
C: ASPECT RATIO The forward slash separates the tire width number from the two-digit aspect ratio. The bigger the aspect ratio, the higher/taller the tire’s sidewall, or “profile” as it’s sometimes called.
The aspect ratio is indicated on the tire sidewall as a percentage. It’s the height of the sidewall measured from wheel rim to top of the tread, expressed as a percentage of tire width.
In this example, the aspect ratio is 65, meaning the sidewall is 65 percent as high as the tire is wide. To get the sidewall height, take the tire width of 215 mm and convert it to inches (8.46). Then multiply this by 65% (.65). This gives you an answer of 5.5, the sidewall height in inches.
D: CONSTRUCTION TYPE This single letter tells you about the internal construction of the tire.
R is for radial tires, the industry standard for most tires today. They have better road grip, lower rolling resistance for better gas mileage, ride comfort and durability than previous generations of tires.
In a radial tire, the plies — layers of strong cords made of a blend of polyester, steel and fabric and coated with rubber — are laid perpendicular to the direction of travel.
D is for tires built with diagonal (crisscrossed) plies, called bias-constructed tires. They are also called conventional, x-ply, or cross-ply tires. Some motorcycle and trailer tires still use this internal construction.
Some run-flat tires are identified with an F followed by the type of internal construction.
E: WHEEL DIAMETER This two-digit number specifies wheel diameter in inches. It’s the distance between the two bead seat areas (where a tire gets tightly sealed onto the wheel).
F: 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. In our example, the load index is 89, which indicates the tire has a load capacity of 1,279 pounds, when inflated to the tire’s maximum air pressure rating.
G: SPEED RATING The last letter is the tire speed rating. This indicates the top speed it’s safe to travel at for a sustained amount of time. A tire with a higher speed rating can handle heat better and provide more control at faster speeds. The maximum operating speed of a vehicle is no more than the lowest speed rating of all tires mounted on the vehicle. (Of course, you should always abide by speed limits for safer driving.) Speed rating is usually, but not always, a single letter (see the chart).
Below you will find several charts that will help you understand tire sizing numbers, including a load index chart and speed rating chart.
A tire size calculator is a quick way to see whether the tire size you’re considering will likely fit your car, SUV, sports car, light truck or crossover.
But remember that is only an estimate. It’s important to stay within the sizing tolerances of your vehicle.
Tires that are the wrong size could cause some pull in the steering wheel, rub against the suspension or body of your vehicle, reduce clearance on hills, or result in a stiffer or noisier ride.
If you’re considering mounting a different tire size on your vehicle, check with a tire expert. Find out whether the tires and wheels you have your eye on are the right fit for your vehicle’s suspension, gearing, and bodywork. And ask how any differences in revolutions per mile, tire speed, load index, and speed rating will affect your ride quality and vehicle performance.
See how new tires and rims will look on your car or truck using our Virtual Wheels simulator, available at any Les Schwab.
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JEGS Tech Specialist
Pete Mitchell
How To Read A Tire SizeBeing able to read and understand a tire size can be very helpful.
On the sidewall of the tire you will find a tire size (for example, “P235/60R15”). We will use this example to decode the sizing. The letter at the beginning denotes the vehicle type. “P” means “passenger tire”. You will also find “LT” for light truck tire and other prefixes. The number following it is the tire width. This is measured across the top of the tire and is in millimeters. So in the example, the tire is 235 millimeters wide, across the top. The next number is the aspect ratio. This represents the sidewall height (from the top of the wheel to the top of the tire) as a percentage of the width. In our example, “60” means that 60% of the width (235mm) is the distance from the top of the wheel to the top of the tire. In this case, that would be 141mm. The lower this number is, the skinnier the tire will look from the side. Often referred to as a “low profile” tire, these are typically on late-model vehicles and sport cars. The higher this number is, the thicker the tire will look, typically on trucks which have high profile sidewalls.
The next digit is a letter. In this example, “R” for radial tires. The final number is the wheel diameter, in this case, a 15” diameter wheel will fit this tire.
To measure the diameter of the tire, set it upright on a level flat surface and with a measuring tape, measure from the bottom to the highest point at the top of the tire. This will give you the diameter. If you want to calculate the tire diameter based on the tire size (though doing a web search will be typically quicker), you can do so with the following example. A P235/60R15 tire is 235 millimeters in width, with 60 % of the width being the distance from the top of the wheel to the top of the tire; in this case, 141 millimeters. 141 millimeters converts to 5.55”. This is the sidewall height. If you add this to the wheel diameter, in this case 15” and then the sidewall height for the other end of the tire, 5.55”, you will get your total tire diameter, which would be approximately 26.
1” tall.
To measure the width of a tire, simply measure across the top of the “footprint” the tire would make. You can also determine the width by the first number in the tire size. For example, with a 235/60R15 tire, the width of the tire would be 235 millimeters or approximately 9.25 inches.
How To Measure Tire Sidewall HeightTo measure the sidewall height of a tire, begin your measurement at the top of the wheel and go up to the highest point of the tire. This will tell you your sidewall height. You can also calculate the sidewall height based on the tire size read from the sidewall. For example, P235/60R15 tire size will be 235 millimeters across the top of the tire. The sidewall height is 60% of the tire width, in this case 141 millimeters or 5.55”.
What Is Tire Section Width And How Is It Measured?Tire section width refers to the tire sidewall distance from where it balloons or bulges out from the furthest point on each side.
Tire section width can be very important when clearance is tight in the vehicle's wheel well. This measurement helps to ensure there is plenty of room from turning the steering wheel or due to compression of the front or rear suspension. One way to measure tire section width is by laying a mounted tire with the correct tire pressure on its side. Then measure up to the top of the sidewall using a level and straight edge.
A tire speed rating is the maximum speed the tire can safely be used. Beyond the speed rating, the tire will not have the control, stability, or handling it was designed to be used for. The speed rating can be found after the tire size on the sidewall of the tire designated by a letter. Typical passenger car tires will have a speed rating letter from L to Y, ranging in speed from 75 mph to 186 mph. Beyond 186 MPH, the speed rating starts with the letter “Z”. The following is a sample list of speed rating codes and their maximum rated speed:
R = 106 MPH
S = 112 MPH
T = 118 MPH
U = 124 MPH
H = 130 MPH
V = 149 MPH
W = 168 MPH
Y = 186 MPH
Z = Over 186 MPH
What Is The Difference Between A Bias-ply Tire And A Radial Tire?Since the late 1970s, radial tires have been the most common tire for passenger vehicles.
Prior to that, bias ply tires were the standard style. Bias ply tires have individual cords that run in layers that are opposing angles or in “bias”, giving to the name of the tire. In contrast, radial tires have steel cords or “belts” to make the tread surface tougher and more puncture proof than a bias ply tire. Radial tires also tend to keep the vehicle running in a straight line on the road where bias ply tires can tend to pull a vehicle into ruts or uneven surfaces while driving. Radial tires also do better for performance handling during cornering. This is due to the thinner sidewall a radial tire has, allowing it to flex better. Bias ply tires tend to do better with larger vehicles and with bigger weight loads as they have thicker sidewalls. Radial tires also dissipate heat faster and better than bias ply tires, which is desired as heat greatly affects tire life.
Tires are made with different compounds for use in varying applications.
Each has its advantages and disadvantages. For applications where long life and mileage is the goal, a harder compound is going to wear less quickly than a softer compound. However, harder tires tend to have less traction than softer tires. For drag racing applications, softer compounds tend to do better as they will grip the surface more than hard tires. This isn’t always the rule as in some applications, harder compound tires actually will do better under certain conditions. Soft compounds also wear faster as they will shed the rubber more quickly.
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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
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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, acceleration and braking speed and many other characteristics will change, which will have to get used to again.
In simple terms, tire profile height is the distance from the rim to the wheel tread surface.
The height of the tire profile in metric size is marked on the wheel as a value that indicates a percentage of the tire width. For example, the dimension 215/65 R16 indicates that the height of the tire profile is 65% of the width of 215 mm and is 139.75 mm, with a wheel radius of 16 inches.
The metric dimension is most common among the assortment of car tires in stores, so it will not be difficult to calculate the height of the tire profile. To convert metric to inch: 1 inch = 2.54 cm.
Inch tire dimensions do not show the profile height in the marking, the overall wheel height, wheel width and tire radius are indicated in inches, for example, 31x10.5 R15. Tire conversion from inches to metric. To calculate the metric profile height, it is necessary to subtract its radius from the total wheel height, divide the resulting value of the total tire profile height in half and get the profile height. (31-15): 2=8" profile height, now 8x2.54cm=20.32cm metric profile height.
As a rule, car manufacturers recommend the optimal tire size for each particular car model and modification, with limits to possible deviations in size. However, practice shows that even going beyond these dimensions can improve the driving performance of the car for each specific case, depending on the operating conditions and the requirements of the owner.
When changing the standard size of a tire, it is necessary to understand what the height of the tire profile affects in order to be prepared for changes in the behavior of the car and possible consequences. By increasing or decreasing the profile of a tire, indicators such as stiffness, comfort, load and reliability change.
Increasing the tire profile within reasonable limits (1-2 sizes without increasing the width), the car will become softer and more comfortable, due to the larger diameter of the wheel, the car's landing will increase and its control will practically not worsen. If you exceed the allowable height of the profile, then the car will have a roll in control, deterioration in braking and cornering, due to an increase in the center of gravity and a crease of the rubber along the height of the profile, an increase in fuel consumption, possible grazing on the elements of the body and suspension at the moment of complete inversion of the steering wheel or at full load.
And also, there will be an additional load on the chassis parts, which will reduce their service life.
Reducing the profile of a car tire without changing the width and radius will allow you to get better handling and improved braking. The lower the rubber profile, the less comfort, the car becomes stiffer, the load on the chassis parts increases, the chances of a wheel breakdown from hitting the disk and disk breakage increase due to the reduced tire donut, fuel consumption increases due to a decrease in tire diameter (fixed by increasing the wheel radius ). Moving along the path of reducing the profile and increasing the radius to equalize the overall diameter of the wheel, fuel consumption will remain without an increase, however, the rest of the nuances of low-profile rubber operation will remain.
Car owners are recommended:
