Tyre sizes and markings: the answers

The code shows width, profile, construction, rim diameter and two performance codes. In 205/55 R16 91V: 205 is the tread width in millimetres, 55 is the sidewall height as a percentage of the width (so about 113 mm), R stands for radial construction, 16 is the rim diameter in inches, 91 is the load index and V the speed code.

Sources: en.wikipedia.org

The profile, also called aspect ratio, is the sidewall height expressed as a percentage of the width. In a 225/45 R17 the sidewall is 45% of 225 mm high, that is about 101 mm. The lower the number, the lower the sidewall: handling is more responsive, comfort is lower and the rim is less protected from potholes.

Sources: en.wikipedia.org

They are the codes for the casing construction. R indicates a radial tyre, the current standard for cars and vans. D or the dash (-) indicate a bias-ply (cross-ply) tyre, rare today and used mainly on motorcycles, classic vehicles and special vehicles. B indicates a belted bias tyre. ZR is an old notation that put the speed in the size and indicates suitability for over 240 km/h: today the actual speed is read from the speed code that follows the load index.

Sources: en.wikipedia.org

The C stands for Commercial: it is a tyre for vans and light commercial vehicles, with a sturdier construction and higher loads than a car tyre of the same size. It usually carries a dual load index, such as 107/105T: the first applies to single fitting, the second to twin (dual) fitting. It generally runs at higher pressures. On a vehicle that requires C tyres, you must not fit a car tyre with a lower load capacity.

Sources: en.wikipedia.org

The date is in the last four digits of the code that begins with DOT: the first two are the week, the last two the year. For example 2324 means week 23 of 2024. The full code also includes the plant and size identifier. For tyres made before 2000 only three digits were used. The code may be printed on one sidewall only, sometimes the inner one.

Sources: en.wikipedia.org, michelin.ca

There is no single expiry date: it depends on how the tyre has been used and stored. Michelin recommends having tyres inspected by a professional at least once a year after five years of use and, as a precaution, replacing them ten years after the production date, regardless of the remaining tread; the same applies to the spare wheel. Over time the materials degrade and cracks may appear on the sidewall. The age is read in the last four digits of the DOT.

Sources: michelin.ca

M+S (Mud and Snow) is a marking that indicates a tread pattern designed for mud and snow, but it is not tied to a snow performance test: according to Continental it is in practice of little significance, and it also appears on tyres not suited to winter. It is also found on many all-season tyres. To get verified snow performance, look for the three-peak mountain snowflake symbol (3PMSF), awarded only after a snow grip test.

Sources: european-rubber-journal.com, legislation.gov.uk, en.wikipedia.org

The 3PMSF (Three-Peak Mountain Snowflake) is the pictogram of a three-peak mountain with a snowflake. It indicates that the tyre has passed a snow grip test under UNECE Regulation No. 117 (annex 7), reaching a minimum index. It is the same symbol shown on the EU tyre label for snow. It is the most reliable sign for recognising a winter or all-season tyre with verified snow performance.

Sources: legislation.gov.uk, energy-efficient-products.ec.europa.eu

TWI stands for Tread Wear Indicator. On the sidewall there are small triangles, or the manufacturer's logo, marking the position of small raised rubber bars inside the main grooves. When the tread wears down to the level of the bars, the tyre is at its wear limit. They let you check wear at a glance, without tools.

Sources: en.wikipedia.org

Run-flats have reinforced sidewalls that let you keep driving for a limited distance after a loss of pressure. Each brand uses its own marking: RFT (Bridgestone), SSR (Continental), ZP (Michelin), ROF and DSST (Goodyear/Dunlop); the marking RSC instead indicates the BMW-approved run-flat system. For its SSR tyres, Continental states a maximum of 80 km at 80 km/h and requires the tyre pressure monitoring system (TPMS). The exact limits are those given by the tyre and vehicle manufacturer.

Sources: continental-tires.com, en.wikipedia.org

They are original equipment markings: they indicate that this version of the tyre was developed and approved with a carmaker. For example MO for Mercedes-Benz, AO for Audi, the star (*) for BMW/Mini and N0, N1, N2 and so on for Porsche. A marked tyre can also be fitted to other cars if the size and indexes are correct; on the car it was developed for, it helps maintain the characteristics set by the vehicle manufacturer.

Sources: en.wikipedia.org

It is the type-approval mark under the UNECE regulations. The capital E in the circle is followed by the number of the country that issued the approval: for example E1 Germany, E2 France, E3 Italy, E4 Netherlands. Next to it appears the approval number. For tyres the main regulations are No. 30 for cars, No. 54 for commercial vehicles and No. 75 for motorcycles and mopeds.

Sources: en.wikipedia.org, en.wikipedia.org

They are the grades of the US UTQG system, introduced by the federal agency NHTSA for car tyres: treadwear is a relative durability number, traction a letter (AA, A, B, C) for wet braking, temperature a letter (A, B, C) for heat resistance. Excluded, among others, are dedicated winter tyres and light truck tyres. The treadwear value is assigned by the manufacturer and cannot be rigorously compared between brands. In Europe the reference is the EU label.

Sources: en.wikipedia.org

It is best to fit only sizes provided by the vehicle manufacturer for that model, listed in the car's documentation. There are often several alternative sizes provided, with a very similar outer diameter and suitable load and speed indexes. The tyre sidewall tells you what is fitted, the vehicle documentation shows the intended sizes: compare them before buying. For the legal aspects of sizes different from those indicated, check with a professional.

A size is approved for a vehicle when the manufacturer has tested and approved it for that model and it appears in the vehicle's documentation. A tyre can be on sale in a size that is not intended for your car. An approved size guarantees compatibility with wheel arches, steering, brakes, load and the vehicle's maximum speed. On our site the size finder shows the sizes by car or motorcycle make and model, with the original load index and speed code where a source states them.

It is possible if the rim and tyre are specified by the manufacturer for your model. The overall diameter of the wheel must stay practically the same as the original, so as not to alter the speedometer and driver-assistance systems: increasing the rim reduces the sidewall height. The load index must not fall below the specified one. Typical effects are more precise steering, less comfort over potholes and a greater risk of rim damage.

Sources: en.wikipedia.org

It depends on the outer diameter of the wheel. If the new tyre has an almost identical diameter to the original, the speedometer does not change appreciably. If it is larger, the real speed exceeds the indicated one; if it is smaller, the opposite happens. UNECE Regulation No. 39 states that a speedometer must never indicate less than the real speed, so the normal deviation goes towards a slightly higher indicated value.

Sources: en.wikipedia.org

A directional tyre has a V or arrow tread pattern and only one direction of rotation, shown by an arrow on the sidewall with the word ROTATION: it must turn in that direction to clear water well. An asymmetric tyre has different patterns on the inside and outside and a fixed side, marked INSIDE and OUTSIDE. A tyre can be both directional and asymmetric: fitting must respect both indications.

Sources: en.wikipedia.org

The word OUTSIDE must face the outside of the car and INSIDE the inside, once fitted on the rim. An asymmetric tyre cannot be fitted the wrong way round: the outer and inner patterns have different jobs, for example cornering grip on one side and water clearance on the other. If the word OUTSIDE is on the visible side of the wheel mounted on the vehicle, the fitting is correct. That is why the tyre fitter must check the side before fitting.

Sources: en.wikipedia.org

You cannot cross them simply by moving them from one side of the vehicle to the other: a directional tyre must turn in the direction shown by the arrow. You can swap the wheels between the front and rear axle on the same side, keeping the direction of rotation. To move a tyre to the other side it has to be demounted from the rim and refitted the other way round. If front and rear sizes are different, swapping between axles is not possible.

Sources: en.wikipedia.org

They are called staggered setups: the rear wheels are wider or have a larger diameter, usually on powerful rear-wheel-drive cars to improve traction, or for style reasons. The sizes for each axle are specified by the manufacturer. The price is less maintenance flexibility: tyres cannot be swapped between front and rear and a tyre from one axle cannot replace one from the other.

Electric cars are often heavier because of the battery, so the tyres must bear higher loads. The marking HL (High Load), before the size, indicates a tyre under an ETRTO standard that, for the same size and pressure, carries more load than an XL. If the car came with HL tyres, they must be replaced with HL. Other markings, such as EV or ElectricReady, are individual manufacturers' brands, not standards: to find out what they offer, check the model's datasheet.

Sources: michelin.com.au

Yes, but the markings are not a standard: each manufacturer uses its own wording. They usually indicate a tyre with a layer of sound-absorbing foam inside, which reduces perceived noise, or with an internal sealant that closes small tread punctures. To find out which technology is present and whether it is available in your size, check the manufacturer's datasheet for the specific model.

XL (Extra Load) or Reinforced indicates a tyre with a reinforced structure that, for the same size, has a higher load index than a standard tyre and reaches that load at a higher reference pressure. An XL can replace a standard tyre of the same size; if the vehicle manufacturer specifies an XL, it must not be replaced with a tyre with a lower load index. The operating pressure remains the one indicated by the car manufacturer.

Sources: en.wikipedia.org

A tubeless tyre carries the word TL (Tubeless) on the sidewall, one with an inner tube TT (Tube Type). Modern car and van tyres are tubeless. On motorcycles and bicycles both types exist and the rim must be suitable. An inner tube must not be fitted in a tubeless tyre unless the manufacturer allows it.

Sources: en.wikipedia.org

The T before the size (for example T125/70 R16) indicates a tyre for temporary use, the so-called space-saver. It is narrower than a normal tyre and is meant to get you to a tyre shop, not for prolonged use. The manufacturer states the maximum speed allowed with the space-saver fitted, usually on a label or in the manual. It must be replaced with the normal wheel as soon as possible.

Sources: en.wikipedia.org

On many tyres, particularly those also sold in the United States, the sidewall shows MAX LOAD, the maximum load of the single tyre, and MAX PRESS, the maximum pressure allowed. The maximum load corresponds to the load index (for example 95 = 690 kg). The maximum pressure is not the operating pressure: the correct pressure is the one given by the vehicle manufacturer, on a plate (often on the door pillar or the door) or in the owner's manual.

Sources: middle-east.michelin.com, law.cornell.edu, en.wikipedia.org