When it comes to finding out information about your tyres, whether it’s the tyre diameter or the overall size, this can all be found on the tyre’s markings. If you’re not sure where to look, or how important this information is, let our guide shed some light on the matter.

Where do i find the tyre size?

The tyre size can be found on the sidewall of every car tyre. It is written as a sequence of numbers and letters in a set order, such as 195/60 R15.

This same sequence can be found in your car’s manual, usually under the technical specifications. Here, the number given represents the correct size for your vehicle, as recommended by the manufacturer.

How To Choose The Right Tyre Size?

In addition to ensuring the tyre size is right for your vehicle, there are a few additional things you need to remember:

  • The load index needs to be high enough for your vehicle.
  • The tyres should also not be too narrow, as this can impact the car’s grip and stability in corners.
  • Likewise, the tyre should also not be too wide, as this can negate other dynamics, as well as causing the car wheel to rub against the wheel housing.
  • The tyre diameter is also important, as an incorrect size can distort the readings given by both the odometer and speedometer. In extreme cases, this can lead to quicker degradation of the car suspension system and transmission elements.
  • You should also check if the width, diameter and type of rims on your vehicle will accommodate any given tyre size.

How to select  alternative tyres>

If you’re looking to choose an alternative tyre to the original equipment supplied with the vehicle, there are a range of aftermarket tyres available. However, when it comes to ensuring the right diameter, there is a general rule that the external diameter should not be exceed the original tyre size by more than 1.5% and it should not be smaller than 2%.

tyre-size-3-tyre

Be sure to check the tyre diameter of any aftermarket tyres you use.

What Standards Should A Tyre Meet?
 
When buying new tyres in Europe, any marketed product needs to meet the following standards:

  • All tyres must meet the standards set out in Regulation 30 of the United Nations Economic Commission for Europe (UNECE), specifically under “Tyres for motor vehicles and their trailers”.
  • Tyres must also be approved by a certified body, such as UTAC or TUV.

In Europe, the standardisation of tyres and rims is handled by the European Tyres and Rims Technical Organisation (ETRTO), which defines the standards for tyre sizes as well as the dimensions for these given sizes. They also oversee the wider range of tyre markings displayed.


tyre-size-1-01

An example of a tyre size marking.

Why were all tyre and rim sizes standardised?

Tyres use a standardised system to determine and display sizes. In addition to making this easier to customers to understand, This includes:

  • Limiting the number of available tyre types and sizes.
  • To ensure the whole automotive market is covered by one system
  • To prevent automotive and tyre manufacturers from creating their own proprietary sizes
  • To make tyres easy to replace across brands and ranges.

Why is the sidewall height (tyre profile) expressed as a ercentage?

The tyre profile - the height of the sidewall - is not expressed as an exact number or measurement. Instead, it is given as a percentage and this is for both historical and practical reasons. In the past, the sidewall height did not need to be given it is close to its width. As the market began to develop, the contact patch increased, due to different tyre widths, forcing manufacturers to also include the cross-section (profile) of the tyre.

tyre-size-4a-09

The evolution of tyre profile over the last decades.

Because of the changing size, the profile percentage is the most practical way to display this information. This way, even a small change in increment can be help describe tyres of different types, dividing the whole tyre range into a dozen or so size groups.

Why does the tyre width always end in 5?

Unlike the profile, the tyre width is an absolute value. The minimum width is 95 mm and it goes up in increments of 10 mm. This is why the width always ends in 5.

Why is the rim size give in inches?

If the width is expressed in millimetres, it is only natural that drivers question why the rim size is given in inches.
 
Rim sizes have always been given in inches, as this keeps the numbers consistent. Changing the rim size to a metric system would mean the numbers were inconsistent. The same would also happen changing from a metric to imperial tyre size. To ensure the numbers are in an orderly fashion, it is necessary to use two different systems.

how to calculate external tyre diameter?

The external diameter is the absolute tyre height, from the ground to the upper edge of the tyre (the surface), expressed in millimetres.

tyre-size-5-09

Tyre dimensions.

It can be calculated based on readily available information using the following formula:
 

d = dr+2*h = dr+2*Sn*ar where:

  • d - external diameter of a tyre
  • dr – nominal rim diameter
  • h – tyre profile
  • Sn – nominal width of the tyre
  • ar – profile height

As an example, let’s calculate the external diameter for a 195/65 R15 size tyre.

  • The rim diameter (dr) is 15
  • The normal width (Sn) is 195
  • The profile height (ar) is 0.65

The external diameter (d), then, is 15 + 2*195*0.65. This equals 634,55, so we can conclude the external diameter for a 195/65 R15 tyre is 634.5 mm.

Why is the external diameter an important parameter?

The external parameter is very important and it is especially crucial when buying aftermarket or replacement products.

tyre-size-6-09

The external diameter is especially important for drivers buying aftermarket tyres.

If you ever change our tyre or rim size, the resulting product should still have the same external diameter as the original tyres. As a rule, the different should not be smaller than 2 per cent of the original diameter, or larger by 1.5%.

Why do tyres and rims keep getting bigger?

As modern vehicles become faster and more powerful, they require more efficient braking systems. Consequently, this requires bigger wheels and tyres.

When it comes to the tyre industry, there are two compatible sizing systems currently in use:

There are two compatible tyre sizing systems currently in use in the world:

  • The European metric system
  • The American p-metric system
tyre-size-2-0

The Eurometric system is commonly used all over the world. A typical tyre size is written like this: 195/65 R15 91T

There are, however, still a few metric-system tyres with profiles of 80 and 82. These so called “full-profile” tyres typically omit the profile altogether, such as “155 R13” tyres. In this example, these would be equivalent to 155/80 R13 tyres.

Furthermore, if a eurometric tyre has a “C” at the end, it means the product is designed for a commercial vehicle. So a 195/70 R15 C tyre is, in essence, the same as a 195/70 R15, aside from its intended purpose.

tyre-size-8-01

The tyre sizes on American tyres is usually marked differently from European tyres.

The North American P-metric system, is the main alternative to marking tyre sizes. It was first introduced by American tyre manufacturers in 1977 and typically looks like this: P205/80 R14 93 S.

If the size begins with a “P”, it marks the tyre as fit for passenger cars, as well as minivans, SUVs and small pickup trucks. “LT”, on the other hand, signifies products for light trucks.

Other than this, this system appears similar to the European system, so the equivalent to an European “C” might be something like LT 245/75 R16 108/104S.

Imperial tyre sizes

In the past, the “imperial sizing” was used and, today, this is only used for historical vehicles and vintage car tyres. Under this system, sizes were either given in inches or alphanumerically.

For the former, such as 6x13, the first number was the width, while the second referred to the rim diameter. Both numbers were in inches and any profile information was given.

Other tyre sizing systems

Spare wheels have a T in their marking, labelling them as a temporary spare. So a T125/90 D16 98M tyre is specifically designed as a temporary product.
 
Professional rally tyres (used in motorsport) have different markings depending on the manufacturer, different from those found in civilian systems. The rim diameter, for example, is sometimes given next to the width, as this information can be quite relevant in the heat of a pit stop change.

tyre-size-8-08

Motorsports products use different tyre markings.

Let’s look at some example markings used in this system, as there can be a huge difference in the race tyre markings.

 

Michelin tyres have the following markings - 20/65-18 X TL - where:

  • 20 refers to the tyre width in centimetres (in this case, it is actually 210 mm)
  • 65 refers to the external diameter, in centimetres
  • 18 is the rim diameter in inches
  • X marks the tyre as a radial product.
  • TL marks the tyre as a tubeless product.

BFGoodrich tyres, on the other hand, have a marking system similar to civilian ones. A typical example might be 200/50 17, where:

  • 200 refers to the tyre width in millimetres.
  • 50 is the percentage ratio between the profile and width
  • 17 is the rim diameter in inches.

Other manufacturers use millimeters for both the width and diameter. The size marking might look like this - 185/510 R13 - where:

  • 185 is the tyre width in millimetres
  • 510 is the external diameter in millimetres
  • 13 is the rim diameter

Some English companies, such as AVON tyres, may use a similar marking, but with all the figures expressed in inches, such as 6.0/21.0-13.