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Tire Specs Explained


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The purposes of the Tire and Rim Association, Inc., include the establishment of interchangeability standards for tires, rims and allied parts for the guidance of manufacturers of such products, designers and manufacturers of motor vehicles, aircraft and other wheeled vehicles and equipment, and governmental and other regulatory bodies.

The Tire and Rim Association in the United States has established a liaison with the following international tire and rim organizations:

  • Associacao Latino Americana De Pneus E Aros
    San Paulo, Brazil
  • European Tyre and Rim Technical Organization (ETRTO)
    Brussels, Belgium
  • Indian Tyre Technical Advisory Committee
    New Delhi, India
  • The Japan Automobile Tire Manufacturers' Association (JATMA)
    Tokyo, Japan
  • The Tyre and Rim Association of Australia
    Hawthorn, Australia

Maximum Load

A tire's maximum load is the most weight the tire is designed to carry. Since a tire's load carrying capacity is related to the tire's size and how much inflation pressure is actually used, maximum loads are rated with the tire inflated to an industry assigned inflation pressure.

Additionally, load ranges are used to separate tires that share the same physical size, but differ in strength due to their internal construction. 'Higher' load ranges are used to identify tires that have a stronger internal construction, and therefore can hold more air pressure and carry more weight.

Each load range has a assigned air pressure identified in pounds per square inch (psi) at which the tire's maximum load is rated. Listed below are the air pressures at which maximum load is rated for popular P-metric and LT tires:

Tire Load RangesInflation Pressure Assigned
For 'Maximum Load' Ratings
P-metric
Standard Load(SL)35 psi
Extra Load(XL)41 psi
Light Truck
Load Range C(LRC)50 psi
Load Range D(LRD)65 psi
Load Range E(LRE)80 psi

P-metric tires used on passenger cars and station wagons are rated to carry 100% of the load indicated on the tire's sidewall (or listed for the tire in industry load/inflation charts). However, if the same P-metric tires are used on light trucks, (pickup trucks and sport utility vehicles for example), their carrying capacity is reduced to 91% of the load indicated on the tire's sidewall. This reduction in load results in causing light truck vehicle manufacturers to select proportionately larger P-metric sized tires for their vehicles to help offset the forces and loads resulting from a light truck's higher center of gravity and increased possibility of being occasionally 'overloaded.'

For example, P235/75R15 P-metric sized, standard load tires used on cars and light trucks would be rated to carry the following maximum loads at 35 psi:

CarsFull Value2028 lbs.
Light Trucks9% Reduced Value1845 lbs.

Additionally, while a tire's maximum load is the most weight the tire is designed to carry, its load carrying capacity at lower inflation pressures is proportional to how much inflation pressure is used. For example, P235/75R15 P-metric sized, standard load (SL) and extra load (XL) tires used on cars would be rated to carry the following loads at the inflation pressures indicated:

Air Pressure (psi)2023262932353841
P235/75R15 SL154316351753185219402028

P235/75R15 XL15431635175318521940202821052183

Note: 35 psi is the assigned 'maximum load' pressure for standard load tires and 41 psi is the assigned 'maximum load' pressure for extra load tires.

The above chart correctly shows that an extra load tire is not rated to carry any more load than a standard load tire when both are inflated to the same pressure (up to the standard load tire's 'maximum load' pressure of 35 psi). This is because a tire's load capacity is a function of its size (which determines the size of the 'air chamber'), its construction (which determines how much pressure can be held) and the actual air pressure used (which determines how many air molecules are forced inside the chamber). All tires with equivalent physical dimensions carry equivalent loads (until they reach their maximum load pressure).

The tire's maximum load is indicated in relatively small sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum load capacity is branded on the tire in kilograms (kg) and pounds (lb.). These values can also be found in the industry's tire load & inflation charts.

NOTE: P-metric and Euro-metric sized tires' 'maximum load' inflation pressure may be, and often are, different that the tire's 'maximum inflation pressure'.

Maximum Inflation Pressure

A tire's maximum inflation pressure is the highest 'cold' inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. It is also important to remember that the vehicle's recommended tire inflation pressure is always to be measured and set when the tire is 'cold'. Cold conditions are defined as early in the morning before the day's ambient temperature, sun's radiant heat or the heat generated while driving have caused the tire pressure to temporarily increase.

For the reasons indicated above, It is also normal to experience 'hot' tire pressures that are up to 5 to 6 psi above the tire's recommended 'cold' pressure during the day if the vehicle is parked in the sun or has been extensively driven. Therefore, if the vehicle's recommended 'cold' inflation pressures correspond with the tire's maximum inflation pressure, it will often appear that too much tire pressure is present. However, this extra 'hot' tire pressure is temporary and should NOT be bled off to return the tire pressure to within the maximum inflation pressure value branded on the tire. If the 'cold' tire pressure was correctly set initially, the temporary 'hot' tire pressure will have returned to the tire's maximum inflation pressure when next measured in 'cold' conditions.

A tire's 'maximum inflation pressure' may be different that the assigned tire pressure used to rate the tire's 'maximum load'. For example, while a P-metric sized standard load tire's maximum load is rated at 35 psi, many P-metric sized standard load performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly). This additional range of inflation pressure (in this case, between 36 and 44 psi) has been provided to accommodate any unique handling, high speed and/or rolling resistance requirements determined by the tire and vehicle manufacturers. These unique tire pressures will be identified on the vehicle placard in the vehicle's owner's manual.

The tire's maximum inflation pressure is indicated in relatively small-sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum inflation pressure is branded on the tire in kilopascals (kPa) and pounds per square inch (psi). These values can also be found in the industry's tire load & inflation charts.

Tread Depth

Tread depth is a vertical measurement between the top of the tread rubber to the bottom of the tire's deepest grooves. In the United States, tread depth is measured in 32nds of an inch. Because it is difficult to accurately measure tread depth with a ruler, tread depth is best measured with a tire tread depth gauge. When tires have neared the end of their life, a U.S. Lincoln penny can also be used to confirm the tire's tread depth. If Lincoln's entire head is visible, the tire is worn to approximately 2/32' and is considered legally worn out in most States.

Average new tires used on cars typically start with 10/32' to 11/32' of original tread depth. Dedicated winter / snow tires and light truck tires typically are deeper (for light truck tires, how much deeper depends on the tire's tread type‚ Highway Rib, Highway All Season, Off Road All Terrain or Off Road Maximum Traction).

As mentioned above, tires are legally 'worn out' In most States when they reach 2/32' of remaining tread depth. For example, a typical tire that starts with 10/32' of original tread depth has only 8/32' of useable tread depth. Its useable tread depth is calculated by subtracting a worn out tire's 2/32' from the new tire's original depth of 10/32'. The final 2/32' of a tire's tread depth isn't part of the equation when it comes to calculating tread depth percentages because the tire is already legally worn out with just 2/32' of remaining tread depth.

Useable tread depth is calculated by subtracting 2/32' from the tires new tread depth. Then usable tread depth is compared to remaining tread depth in order to calculate tread wear percentages. For example, a tire that started with 10/32' of original tread depth and has worn off 4/32' (down to 6/32' of remaining tread depth) is 50% worn

10/32' original tread depth
-2/32' legally worn out tread depth
8/32' of useable tread depth.

8/32' useable tread depth
-4/32' measured tread depth worn away
4/32' of remaining tread depth (50% of 8/32')

A tire that starts with 10/32' of original tread depth has 12.5% wear for every 1/32' that is worn away, and a tire that starts with 12/32' ' of original tread depth, has 10% wear for every 1/32' that is worn away, etc.

Rim Width Range

Because tires have flexible sidewalls, a single tire size will fit on a variety of rim widths. A tire's rim width range identifies the narrowest to the widest rim widths that the tire is designed to fit. The width of the rim will influence the width of the tire. A tire mounted on a narrow rim would be 'narrower' than if the same size tire was mounted on a wide rim. NOTE: Because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.

The industry rule of thumb is that for every 1/2' change in rim width, the tire's section width will correspondingly change by approximately 2/10'.

For example: a tire in the P205/60R15 size is measured on a 6.0' wide wheel and this size tire has an approved rim width range from 5.5' to 7.5' wide. The tire has a section width of 8.23' (209mm) when mounted on a 6.0' wide wheel. If that tire were mounted on all of the rims within its range, the tire's approximate section width would change as follows:

Difference from
Measuring Rim
Rim
Width
Approximate Tire
section width
0.5' narrower5.5'8.03'
Measuring Rim6.0'8.23'
0.5' wider6.5'8.43'
1.0' wider7.0'8.63'
1.5' wider7.5'8.83'

Because of the different wheel widths used in the above example, there is a 8/10' projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. This may affect fenderwell and frame clearances when selecting optional aftermarket wheel and tire packages.

Additionally, some vehicle manufacturers and tire companies have permitted rim widths that are not within the tire's original approved rim width range. For example: BMW has combined 235/40R17 sized tires on 17x7.5' rims (which are 0.5' less than the narrowest 8.0' wide rim listed for the size) on certain M3 models; and Chevrolet has combined P255/50R16 sized tires on optional 16x9.5' rims (which are 0.5' wider than the 9.0' wide rim now listed for the size) on certain Corvette models. While these applications have received the approval of the vehicle and tire manufacturers, staying within the approved rim width range helps assure that the tire's internal stresses are within its design parameters.

Measuring Rim Width

The measuring rim width is the industry standardized rim width upon which the tire must be mounted in order to confirm it meets its dimensional targets. Because the width of the rim will influence the width of the tire, a standard rim width for every tire size is assigned and must be used. This standardized measuring rim width allows all of the tires produced around the world to meet the same dimensional standards and therefore, be equivalent with regards to their physical size. The measuring rim width is sometimes referred to as the tire's 'design rim width'.

The assigned measuring rim width changes with the tire size's section width and with the tire size's aspect ratio. As tire section width increases, the measuring rim width increases proportionately in 1/2' increments. Therefore, relatively narrow wheel widths are assigned for smaller tires while wider wheel widths are assigned for larger tires.

Tire SizeMeasuring
Rim Width
P175/75R145.0'
P195/75R155.5'
P215/75R156.0'
P235/75R156.5'

Additionally, relatively 'narrow' measuring wheel widths are assigned for taller profile tires (75-series sizes) which graduate in 1/2' increments to the wider wheel widths assigned for lower profile tires (40-series sizes)

Tire SizeMeasuring
Rim Width
P225/75R156.0'
P225/70R156.5'
P225/65R166.5'
P225/60R156.5'
P225/55R157.0'
P225/50R157.0'
P225/45R177.5'
P225/40R188.0'


Section Width

A tire's section width (also called 'cross section width') is the measurement of the tire's width from its inner sidewall to its outer sidewall (excluding any protective ribs, decorations or raised letters) at the widest point. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours.

Because a tire's section width is influenced by the width of the rim upon which the tire is mounted, the correct industry assigned measuring rim width for the tire size being measured must be used.

The width of a tire mounted on a narrow rim would be 'narrower' than if the same tire was mounted on a wide rim. NOTE: because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.

The industry rule of thumb is that for every 1/2' change in rim width, the tire's section width will correspondingly change by approximately 2/10'.

For example: a tire in the P205/60R15 size is measured on a 6.0' wide wheel and this size tire has an approved rim width range from 5.5' to 7.5' wide. The tire has a section width of 8.23' (209mm) when mounted on a 6.0' wide wheel. If that tire were mounted on all of the rims within its approved range, the tire's approximate section width would change as follows:

Difference from
Measuring Rim
Rim
Width
Approximate Tire
section width
0.5' narrower5.5'8.03'
Measuring Rim6.0'8.23'
0.5' wider6.5'8.43'
1.0' wider7.0'8.63'
1.5' wider7.5'8.93'

Because of the different wheel widths used in the above example, there is a 9/10' projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. This may affect fenderwell and frame clearances when selecting optional aftermarket wheel and tire packages

Tread Width

The tread width is the distance between the outer edge and the inner edge of the tread of a new tire. However today's radial tires often feature tread designs that incorporate rounded shoulders and there is no industry standard pertaining to 'how much' of the rounded shoulders should be included in the tread width measurement. Because of this, it is difficult to accurately compare the tread width differences of one tire brand to another. Tread width measurements are best used when comparing the various tire sizes or lines manufactured by a single tire manufacturer. Several tire manufacturers have chosen not to publish tread width dimensions.

Overall Diameter

A tire's overall diameter is the outer diameter of the tire measured in the center of the tread. This measurement is made without any load placed upon the tire and after the tire has been properly mounted on its industry assigned measuring rim and has been inflated and reset to its test pressure after 24 hours.

Revolutions Per Mile

Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate.

Country of Origin

Although regularly updated, the country of origin for any tire may vary from the country shown here. Unfortunately, we cannot accommodate requests for tires with a specific country of origin.

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