How to read tire sizes?

  • Author: OPONEO.COM

There are still a few standards of car tire sizes that can be seen on the market. Fortunately, there are not as many them for car tires as there are for bicycle tires! If you were wondering how to read the sizing format on your tires, you came to the right place. In this article, we will describe all of them.

When you are looking for new tires, you can search by providing the website with information regarding your vehicle, or by tire size. There are a few standards of writing the tire size, but you will probably only need one, maybe two, if you are using a light truck. These are the metric size and the inch standard called the high flotation system. Other standards will be described in the end of the article.

  1. P-metric sizing (P255/35ZR18)
  2. Euro-metric sizing (255/35ZR18)
  3. LT-metric sizing (LT265/75R16)
  4. LT High Flotation sizing (31x10.50R15LT)
  5. Other systems

P-metric sizing


P-metric tire size. These tires can also have an XL marking,
indicating an increased load index

Here is an example of a P-metric tire size: P215/60R15. First thing you will notice is the letter “P”.  This is an indicator that this tire is made for passenger cars. On various tires, you may find one of the following indicators:

  1. - (no symbol) - Euro-metric tire, for passenger car
  2. P – passenger car tire;
  3. LT – light truck tire;
  4. T – space-saver, temporary spare tire;
  5. ST – special trailer (utility trailers).

After the letter comes the tire size. The 215 is the width of the tread, in millimeters. This value ranges from about 155 (very narrow tires) up to almost 400 millimeters (15.5” wide Mickey Thompson Sportsman S/R Radial), with the vast majority falling between 155 mm (6.10”) and 285 mm (11.22”).

The slash is a separator, after which comes the height of the sidewall, also called the aspect ratio. This is a percent of the tread width. Therefore, the 60% of 215 mm (8.46”) is 129 mm (5.07”). This means that if you look at the tire from the side, then the height from the tread down to the point where the tire touches the rim is 129 mm.

The letter R will tell you about the tire’s internal structure. Tires are not just rubber – they also have a grid of metal cords inside, called plies, which make the tire more rigid and protect it from damage. If you look at a tire from above, then the radial ply tires, the R ones, have cords that are aligned horizontally one after another. All modern passenger car tires use this construction.

  1. R means the radial-ply internal structure;
  2. – (dash) indicates the bias-ply (sometimes called “cross-ply” or diagonal) internal structure;
  3. D means the diagonal or bias internal structure;
  4. B is used for belted bias construction.

The bias-ply tires are heavier, more resistant to damage and can carry more load than radial tires, but they will not let you go very fast, the ride is uncomfortable, such tires have an impact on maneuverability and wear much faster, so they are used in haulers, tractors, industrial vehicles, which will not go very fast, and on smaller vehicles such as ATV, motorbikes and bicycles. All passenger cars use the radial construction, and therefore will have the R.

The last number indicates the wheel diameter, expressed in inches. The 15 therefore means that this tire will fit a 15” wheel. Modern passenger car, light truck and off-road vehicle tires are usually produced for wheels between 15” and 22” in size, though both smaller (13”, 14”) and larger (even 30”) can be seen on the market.

Low profile tire

Low profile tires, commonly found on passenger cars.

The P-metric sizing system is often accompanied by additional markings. Some of the most popular ones include:

  1. Load range:
    1. LL (light load) - 35 psi maximum pressure
    2. SL (standard load, often omitted) - 35 psi maximum pressure
    3. XL (extra load) - 41 psi maximum pressure
  2. Run-on-flat technology:
    1. ROF - RunFlat technology (unspecified technology)
    2. DSST - Dunlop Self-Supporting Technology
    3. ZP - Michelin Zero Pressure
    4. SEAL - Continental ContiSeal technology
    5. FRT - Federal RunFlat Tire
  3. Sidewall style:
    1. BSW (often omitted) - black sidewall
    2. WSW or WW - white sidewall
    3. OWL - Outlined White Letters
    4. ORWL - Outlined Raised White Letters
    5. VSB - Vertical Serrated Band

Euro-metric sizing

Euro-metric tire sizing format

Euro-metric tire sizing. The basic parameters remain the same,
but the initial letter is absent, and some markings are unique to this system.

Euro-metric system uses the same parameters in designating tire size except they will not have any initial letters. Therefore, a P215/60R15 tire is of exactly the same size as a 215/60R15 tire. The difference between them more relevant for car manufacturers, since American P-metric, LT-metric, T-metric, ST-metric and their equivalent High Flotation sizes are more consistent with formulas used to calculate their carrying capacities. Therefore, American sizing systems make the world an easier place to live for vehicle manufacturers, European ones - not so much. What matters for a typical driver, is that they differ in maximum allowed internal pressure, which can be deducted from another parameter called the load range.

Some markings are unique to Euro-metric tire sizing system:

  1. RF (reinforced) - sometimes used in place of XL. These tires are stronger than P-metric XL tires and can endure up to 42 psi of internal pressure
  2. C (cargo) - light truck tires

Euro-metric tire size

Euro-metric tire size, used with most tires sold outside the US.

LT-metric sizing

LT-metric tires for Light Trucks

LT-metric tire sizes provide all the same measurements except for load range, which will be C, D or E instead of LL, SL or XL.

LT-metric system, again, have all the measurements the same as in P-metric and Euro-metric tires, but these are Light Truck tires. The speed and load indexes can always be viewed in Oponeo, albeit most tire shops will omit them. The LT-metric tires will not have XL or ZR tire markings.

Load range for LT-tires is different than for P-metric or Euro-metric tires. Letters from C to E are most commonly used (with most passenger car tires falling into B category). Load Range rates higher than E (like F, G, H) can be seen on commercial truck tires.

  1. C (6-ply rated) - 50 psi maximum pressure
  2. D (8-ply rated) - 65 psi maximum pressure
  3. E (10-ply rated) - 80 psi maximum pressure
Suzuki Jimny all terrain vehicle

Suzuki Jimny - an offroad vehicle capable of using off-road tires (phot. Suzuki).

LT High Flotation sizing

Light Truck High Flotation tire sizing system

Light Truck High Flotation tire sizing system. It provides tire dimensions in inches and uses different load range coding than P- or Euro-metric tires

The flotation system mostly concerns typical off-road tires, which favor flotation over traction. Traction is important in all tires, however the high flotation-sized ones are especially adapted for one task – not to sink in mud, gravel or sand. They are floating on these surfaces.

Sizing system uses three numbers, all provided in inches. This one is used primarily with larger tires, Nitto Mud Grappler 33x13.50R15 being an example. In this case, all numerical parameters are given in inches. Please note that the “LT” letters indicate this this tire is to be mounted on Light Trucks and is not an indicator of a high flotation tire. There are LT metric-sized tires, such as the new Greenball’s Centennial Terra Trooper A/T 235/85R16LT.

The 33” (83.82 cm) is the diameter of the whole tire. 13.50 means that the tread is 13.5” (342.9 mm) wide. 15” is the diameter of the wheel. The 33x13.50R15LT tire size can be converted to metric tire size system as approximately 343/67R15. First thing you might notice – flotation tires are usually very wide, and have a rather high profile.

Flotation tires are also commonly used on ATV/UTV. For such vehicles, you might see tires with a bias ply internal structure. The R, which normally stands for radial ply structure, is then replaced with a dash. One example is the Greenball Dirt Commander 25x8.00-12 ATV/UTV tire.

Other sizing systems

Apart from the P-metric, Euro-metric, LT-metric and the LT-High Flotation systems, there are several more that you may find when looking at tires for various vehicles. Let us quickly analyze the most common tire size notation system for cars.

Numeric system: 7.00R15LT (radial) or 13-24 (bias). First number stands for the section width. R or the dash stand for the tire construction. Second number stands for the rim diameter. This system is often used with OTR and industrial tires.

Millimeter system: Used with highly non-standard sizes. Two examples are Toyo Proxes RS1 full-slick tire 335/710R18 (with 335 being the tire width and 710 the tire diameter in millimeters, 18 being rim   or Michelin Energy LX4 235/710R460A 104T tire intended to be fit on PAX system wheel with an asymmetric bead.

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