How to Read Car Tyre Sidewall Markings
Tom Vondrasek | 26th May 2024 | 6 minutes to read
The side wall on a tyre contains a lot of information and is often overlooked as it blends into the tyre. Sometimes the brand is highlighted in a different colour, usually white lettering, though this is simply advertising.
Lets breakdown these hieroglyphics so you can decipher what it is and why it is on the tyre:
Tyre Size
This is the dimensions of the tyre and the most vital thing you need to know when buying a new tyre. Without dimensions, you won’t know whether the tyre fits your wheel rim or not.
245/40 R20 – All tyres will have this dimension. Usually in large print. The numbers will naturally vary based on the tyres size. Broken down, it is:
- 245 = Tyre Width - Usually 3 digits and is the width of the tyre in mm
- /40 = Tyre Height - Usually 2 digits and is the tyre side wall height. The number is expressed as a percentage of the tyres width. In this case 40% of 245mm so the height is 98mm
- R = Radial Tyre - Radial tyres are a type of tyre construction. Most tyres these days are Radials
- 20 = Wheel Rim Diameter - Usually in inches. It is the diameter of the the hole in the middle of the tyre.
Some other examples of tyre sizes are:
P 185/75 R14 – Similar to the one above except for the extra letter at the front. 185 is tyre width, /75 tyre height, R is for radial and 14 is wheel rim diameter. This letter P at the front is not always there and lets you know the intended use for the tyre. Some of the different letters you may find are:
- P = Passenger for passenger car
- C = Commercial
- SUV = Sports Utility Vehicle
- LT = Light Truck
- T = Temporary Spare
245/35 ZR19 – Same again except for a letter in front of the R for tyre type. The Z indicates a high-speed tyre capable of over 240Kph. It can also be a V instead of Z indicating a high-speed tyre capable of over 210Kph There can be more details found in the Service Description which follows the Overall Height
Overall Height
Although not written on the tyre, here is how you calculate it.
If you need to calculate the overall height of the tyre from the ground it is:
2 x Tyre Height + Wheel Rim Diameter = Overall Height
Note - As one figure is in mm and the other in inches you will need to convert one of the values so both are in the same measurement.
Service Description
This normally follows the Tyre Size and is 2 digits followed by a letter.
245/40 R20 99Y
- 99 = The first 2 digits are the load index, which indicates the maximum load in Kilograms the tire can carry. You need to look at a load index chart to see what 99 equals which is 775Kg per tyre.
98 = 750Kg and 100 = 800Kg so each increment is 25Kg. - Y = The speed rating, which specifies the maximum speed the tyre can sustain. There is a Speed Rating chart which decodes these letters.
- Y = 300Kph maximum speed which is currently the highest speed rating
- H = 210Kph
- W = 240Kph
Some other examples are:
P 185/75 R14 89H 89 = 580Kg H = 210Kph
245/35 ZR19 98Y 98 = 750Kg Y = 300Kph
Below are 2 examples of common markings you need to know when looking at tyres.
Caption:Common format for tyre size, load & speed rating
Caption:The most important point here is the week & month the tyre was made
DOT Codes
DOT codes or Department Of Transport codes mandatory for tyres in the USA. DOT codes are now found on most tyres around the world.
DOT 6G8X NXVB 0617
- DOT - They all start with DOT Departments Of Transport
- 6G8X NXVB - The next two groups of 4 alpha/numeric characters are to do with the tyre manufacturer. You may find that these 2 groupings may vary in the amount of alpha/numeric characters they have. It may be 3 then 5 instead of 4 and 4. They hold information like country and plant the tyre was manufactured, batch No and other details
- 0617 - It is the last group of 4 numbers and they will always be numbers, that is important. This is the date the tyre was made. The first 2 digits represent the weeks of the year (01 to 52) the last 2 digits are the final digits of the year it was made.
0617 06 = 6thweek 17 = 2017 so this tyre was made on the 6thweek of 2017
Rubber degrades with age and how it is used and stored. Australia currently has no laws on how old a tyre needs to be before it needs replacing. The tyre needs to be roadworthy and a general feeling is that once they reach 10 years or older replacement is warranted.
There are so many variables in tyre life. It may have been stored for 4 years before it was fitted and the vehicle was rarely driven and garaged OR it was fitted within 3 months of being made and the vehicle was driven daily and left out in the weather.
If in doubt get an opinion from your local tyre dealer.
Treadwear, Traction, and Temperature Grades
Another name for this is the UTQG (Uniform Tyre Quality Grading) rating required for tyres sold in the USA. It now does appear on a lot of tyres. There are 3 main points it covers:
- Treadwear – Simply how hard or soft the rubber compound of the tyre is. The softer, the quicker it will wear out, where the harder it is, the longer it will last.
Ratings are from 100 being the softest to 800 which is the most hard wearing. - Traction – This is a measurement of tyre grip on a wet road surface and covers acceleration, braking and cornering in a controlled environment.
Ratings are from AA being the very best to C being the worst. - Temperature – This is the ability for the tyre to resist heating up when in use in a controlled environment. Heat is not good for the tyres rubber compound so the more heat resistant it is, the less it degrades over time.
Ratings are A for the most heat resistant to C for the least
Maximum Load and Maximum Air or Inflation Pressure
These are important maximums and should not be exceeded.
- Maximum Load - is the maximum safe weight, usually in Kilograms, that the tyre can hold up and carry.
- Maximum Air or Inflation Pressure - is how much air pressure, in Kpa (Kilopascals) or PSI (pounds per Square inch) the tyre can safely withstand. Remember this air pressure is not the working air pressure of the tyre. This is best sourced from your vehicles handbook or data plate and is a lot lower.
Other Markings
There are a host of other markings on the tyre, though the main standouts have been explained.
A particular one that deserves a mention is tyre direction of rotation. This is only for tyres that have a tread pattern that is directional, so it is critical they are mounted correctly on the vehicle. There is usually an arrow on the side of the tyre pointing in the direction it needs to rotate for the tread pattern to work.
