How to understand the markings on your farming tyres?
Authored by: Tractor tyre expert | 20 January 2021
It's often when the time comes to change your tractor tyres and choose a new model, that the markings and all the numbers on the sidewalls become important. However, when you fully understand these figures you can improve your tractor’s capacity, for example increasing the maximum load or improving traction… and a change in tyre can also be a chance to boost your machine’s productivity.
However, to make the right choice, it is essential to fully understand all the markings on the sidewalls, each of which is significant. Here is a detailed glossary to help you understand these markings.
1. Why are the markings on tyres not always the same?
Tyres themselves have evolved over time, and the markings with them. Depending on the manufacturers or the geographical area, these markings can differ greatly, while often describing the same things.
The information on tyres indicates the range, size, model, and the casing structure, Radial or Diagonal. There are different tyres for different machines and different uses.
To clarify the markings on tractor tyres, 50 years ago, a European group implemented the ETRTO norm ETRTO (European Tyre and Rim Technical Organisation) whose aim is to simplify and standardise the sizes of farming tyres.
Despite the systematic integration of this norm, there are still tyres in circulation with the old markings, or from other regions in the world where this norm is not applied.
How to read the markings on farming tyres?
Generally indicated in inches, the old markings on tractor tyres are displayed as a string of numbers with a letter or a dash in the middle, as shown in the following examples:
8.3 R 24
12.4 - 28 ou 12.4 X 28
16.9 R 30
The first numbers correspond to the width of the tyre in inches, so a width of 8.3 inches for the first example, 12.4 inches for the second and 16.9 inches for the third (1 inch = 2.54 cm).
The letter R indicates “Radial” and corresponds to the tyre’s internal structure.
The “dash” or the “X” in the second example indicates that the tyre structure is diagonal.
The last numbers numbers in these three examples correspond to the rim diameter in inches, which is respectively 24, 28 and 30 inches (1 inch = 25.4 mm).
Understanding the new ETRTO markings
The majority of farming tyres sold in Europe today carry the European ETRTO markings and must also include the code: E (E3 - E4 - etc…).
It becomes mandatory to have ‘E’ marking on tyres due to changes in EU Vehicle Registration method for original equipment manufacturers, which applies to Agricultural and Forestry vehicles from January 2018.
Number can vary by tyre manufacturer: this depends upon which approved homologation body is used by which tyre manufacture. Bridgestone and Firestone tyres, made in our European factory at Puente Sans Miguel in Spain, are homologated by RDW in the Netherlands, so in this case it will be 4.
The new size marking is composed of a string of numbers, a slash, and letters:
The first numbers indicate the tyre width in millimetres, so 750 mm in width for the first example, 600 mm for the second and 460 mm for the third.
The numbers after the slash indicate the aspect ratio which is the sidewall height in percentage of the section width, so 55% of 750 mm = 412.5 mm sidewall height. This figure corresponds to the series 50, 55, 60, 65, 70, 75, 85. The higher the figure, the greater the height of the sidewall.
The letter R indicates that the tyre’s structure is radial.
The last numbers always indicate the rim diameter in inches (1 inch = 25.4 mm), so 26 inches for the first example, 28 inches for the second, and 38 inches for the last one.
2. Knowing how to adapt the measurements of your farming tyres
Before replacing your old farming tyres, you must understand their measurements to be sure to find the equivalent size with the new markings or to improve your tyres and increase the capacity of your tractor.
The old measurement markings
Although the ETRTO norm has existed since 1964, you can still find tyres with the old markings on non-European tyres or on old tyres used on trailers or other farm vehicle:
12.4 – 28 ou 12.4/11 – 28
10.5/65 – 16
The first numbers indicate the width of the tyre in inches (1 inch = 25.4 mm), or respectively 12.4 inches (12.4 inches x 25.4 mm = 310 mm in width) and 10.5 inches (10.5 inches x 25.4 mm = 260 mm in width).
The last numbers indicate the rim diameter, always displayed in inches, in this case 28 inches and 16 inches.
The number 11 in the first example represents the width of the rim, which can be indicated in addition to the diameter, so 12.4/11 – 28 = The rim measures 11 inches in width x 28 inches in diameter.
The figure 65 in the middle of the second example corresponds to the height of the sidewall in % of the width, as in the ETRTO markings.
In the first example 12.4/11 - 28 the ratio of the height of the sidewall is not indicated. The default sidewall height ratio is 0.82 which gives the following calculation 12.4 x 25.4 x 0.82 = 252 mm in height for the sidewall.
Converting to find the correspondence with the new measurement markings
The new ETRTO markings provide more precise indications on farming tyres than the old markings, as the height of the sidewall is now indicated systematically.
This is an extremely important indication, as the smaller the aspect ratio, the greater the volume of air the tyre can hold: so it can carry a heavier load and increase your tractor’s load capacity.
Here is the conversion using the dimensions in the example shown above:
12.4 – 28 corresponds to the marking: 320/85 R28 or 360/70 R28 or 420/65 R28
10.5/65 – 16 corresponds to the marking: 260/65 R16
The numbers 320 - 360 - 420 correspond to the section (width) of the farming tyre in millimetres.
The numbers /85 or /70 or /65 correspond to the aspect ratio, which can be variable without changing the rim to increase the machine’s load capacity. This number is a percentage of the width, and corresponds to the series 85, series 70 or series 65.
R corresponds to the radial structure.
28 or 16 always correspond to the rim diameter in inches.
How switch from a series to another?
To switch from a series to another, you need to use the tyre correspondence table. The SRI (Speed Radius Index) groups all tyres sizes with similar diameters. Any dimensional transformation requires a lead lag calculation, made by an expert.
3. Indication of tyre category or type
There are several different types of farming tyres and having a thorough knowledge of the different categories available on the market enables you to choose the most suitable model for your needs.
Radial tyre or Diagonal tyre: what’s the difference?
As shown in the previous examples, the tyre structure can be radial or diagonal.
Radial structure (R)
The radial structure is composed of a flexible casing build with several casing plies, as well as several stabilization plies placed on tread area to make it flat and provide lesser tread deformation.
Diagonal structure (D)
The diagonal structure is composed of an assembly of criss-crossing plies which form a thick layer which is sensitive to heating and less flexible than the radial structure.
On the diagonal tyre, the sidewalls and the vertex are part of the same section, they are mutually dependent.
To distinguish between the casings of each tyre type, you just need to look at the pattern of the tread marks:
Radial tread: squared and quite shallow
Diagonal tread: rounded and deeper
There is also a clear difference in the distribution of pressure on the soil:
Radial tyre provides equal ground pressure by dispersing load to the surface. It prevents better soil compaction, improve traction and contributes to time and fuel savings.
Diagonal tyre grounding surface is narrower, leading to the concentration of the load to the centre. It will increase compaction.
Standard tyre, IF tyre and VF tyre: let’s explain
There are three main categories of farming tyres for Radial structures:
Standard Radial structure tyres: also called “traditional radial casing”, these tyres are the most commonly found on tractors and among distributors. To get the most from them you must be able to adjust the inflation pressure according to your driving speed, the load per axle and the type of work required, with low or high torque.
IF tyres (Increased Flexion): more high-tech than standard tyres, they can carry up to 20% more than standard tyres. The casing on these tyres has flexible deformable sidewalls without tread deformation. IF marking will be placed on sidewall just before size marking.
VF tyres (Very high Flexion): These new generation tyres, also called “low pressure tyres” (0.6 bar) can handle a high level of sidewall distortion without damage, with a load capacity up to +40%. Minimum soil compaction or adjusting pressure every time you change ground surface (from field to road). This represents a multipurpose use of pressure. However, the pressure should be increased for intense road use. VF marking will be placed on sidewall just before size marking.
4. Load markings
Each tyre is designed to carry a maximum load defined by the manufacturer. Before buying a farming tyre, it is wise to be aware of the weight of your equipment to avoid any problems down the line. You can find the load index written as a code on the sidewall.
What is the load index?
This is a reference value of the load that a tyre can carry, in relation to a standard pressure of 1.6 bar and a given speed.
The load index generally varies between 100 and 200 as in the following examples:
Correlation table: weight in kilos/ tractor tyre load index
In general the load and speed indexes provided in the different tables are based on a basic pressure of 1.6 bar.
Kg in weight
1 000 kg
1 030 kg
1 060 kg
1 120 kg
1 150 kg
1 180 kg
1 215 kg
1 250 kg
1 285 kg
1 320 kg
1 360 kg
1 400 kg
1 450 kg
1 500 kg
1 550 kg
1 600 kg
1 650 kg
1 700 kg
1 750 kg
1 800 kg
1 850 kg
Kg in weight
1 900 kg
1 950 kg
2 000 kg
2 060 kg
2 120 kg
2 180 kg
2 240 kg
2 300 kg
2 360 kg
2 430 kg
2 500 kg
2 575 kg
2 650 kg
2 725 kg
2 800 kg
2 900 kg
3 000 kg
3 075 kg
3 150 kg
3 250 kg
3 350 kg
3 450 kg
3 550 kg
3 650 kg
3 750 kg
3 875 kg
4 000 kg
4 125 kg
4 250 kg
4 375 kg
Kg in weight
4 500 kg
4 625 kg
4 750 kg
4 875 kg
5 000 kg
5 150 kg
5 300 kg
5 450 kg
5 600 kg
5 800 kg
6 000 kg
6 150 kg
6 300 kg
6 500 kg
6 700 kg
6 900 kg
7 100 kg
7 300 kg
7 500 kg
7 750 kg
8 000 kg
8 250 kg
8 500 kg
8 750 kg
9 000 kg
9 250 kg
9 500 kg
9 750 kg
10 000 kg
10 300 kg
Why it is important to comply with the load index?
As with the speed index, the load index enables you to work in the best conditions. These indications help you to use your tyres optimally and to get the very best from them.
By complying with the load index indicated on your farming tyre, you will increase its lifespan. However, if you don’t comply with the maximum load intended by the manufacturer your tyre will wear down more quickly.
5. Speed markings
The lifespan of a farming tyre depends on several parameters, among which speed is a major element.
What is a speed index?
The Speed Index is defined as the maximum speed authorised for the farming tyre when loaded. In concrete terms it is the ratio of the tyre’s load capacity to the tractor’s forward speed.
Like the measurements, the structure or the load index, the speed index is determined by the manufacturer. It is indicated on the tyre’s sidewall, just after the load index, in the form of an alphabetical code with or without a number:
Speed index table
In general, the load index and the speed index indicated in the different tables are based on a basic pressure of 1.6 bar.
SPEED in km/h
SPEED in km/h
What happens if you disregard the speed index?
Respecting the speed index indicated on your farming tyres will allow you to use them for longer, avoiding the costs of replacing them prematurely.
When you exceed the limits recommended by the manufacturer, you increase the risk of damaging the tyre’s rubber surface, as speed causes excessive heating, making the rubber more flexible and fragile, especially when driving on the road with a heavy load (as road surfaces are highly abrasive).
6. The production date of farming tyres
You can find the production date of your farm tyres easily. This detail will be important if you decide to choose used tyres. Some manufacturers also indicate the place of manufacturing, but this information is not mandatory.
This production date also triggers the five-year manufacturer's warranty. Some manufacturers can extend this warranty, as with Bridgestone Firestone agricultural tyres, to 10 years from the date of purchase of the tyres by the user (subject to presentation of proof, otherwise the 10-year warranty starts on the production date).
The marking system is simple, it indicates the week and then the year of production.
There are 52 weeks in the year. The week is indicated first (from 1 to 52) then the last two numbers of the year.
4899 = 48 and 99 which is the 48th week of 1999
2710 = 27 and 10 which is the 27th week of 2010
3214 = 32 and 14 which is the 32nd week of 2014
0220 = 02 and 20 which is the 2nd week of 2020
In old tyres, you could also find production date marking like “C 41 R”. First letter refers to month, figure in the middle refers to factory location code, and last letter refers to Year.
You can improve your tractor and your productivity simply by changing tyres. Increasing your tyre size or adapt tyre technology will enable you to use heavier equipments while protecting your soils even more.
Respecting the speed and load indexes will allow you to keep your tyres for much longer. Whatever the load, or the speed used, you need to adapt the tyre pressure to your use, based on load and pressure tables provided by each tyre manufacturers.
For more information on tractor tyres
With Firestone, you have the guarantee of tractor tyres that deliver a real advantage, and that in choosing our brand, you can work worry-free.
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This information is intended only to make you aware of the technical and functional aspects of agricultural tires and their use. It does not allow you to make a judgment or a definitive conclusion on a given problem. Only your agricultural tire expert is able to make a technical assessment and take a final decision, case by case.