Choosing the right tyre size for my tractor

Why is the choice of tyre size so vital? The contact patch between the tyres of a tractor and the ground may not seem significant, but this is where the key to vehicle performance lies.
Whether for the transmission of engine power, ensuring good traction or protecting your soil, the choice of tyre has an influence on your productivity, your yield and, at the end of the day, on your profitability.
Do you hesitate between wider tyres, more resistant models or the cheapest option? Would you like to know how to optimise your tractor capacity based on your actual needs?

This comprehensive article explains it all, from the ideal size to match the power of your vehicle to the adjustments necessary to ensure perfect compatibility with your tractor. Discover how a simple change of tyres can improve yields, extend the lifespan of your equipment and optimise your profitability.

1. What width of agricultural tyre to choose?

What is the nominal section width?

The nominal section width of an agricultural tyre indicates its width as measured from one sidewall to the other when not inflated. It’s the first number that you see marked on the sidewall of a tyre.

For example, for a tyre with “520/85 R38” on the sidewall, the “520” corresponds to the tyre width in millimetres.

But bear in mind that on standard tyres, this is not the exact width in contact with the ground, which tends to be less than the width from sidewall to sidewall.

Nominal section width of 520 mm for this standard 520/85 R38 tyre

Factors that influence the effective width on the ground

The level of inflation pressure

The lower the pressure, the more the tyre flattens out on the ground, thus increasing the contact area and allowing the entire width of the tread to be used to the full. This is particularly useful for reducing soil compaction and improving traction.

The tyre fabrication technology

Certain tyre models such as VF tyres are designed to operate at low pressure without compromising the load capacity. These tyres offer a larger soil footprint than standard tyres.

New tyre models such as Bridgestone’s VX-R TRACTOR tyre have been engineered to offer a much more accurate correspondence between the nominal section width and the actual width in contact with the ground.

On the right, the VX-R TRACTOR section width is the actual width in contact with the ground, unlike the standard tyre on the left

What are the advantages of choosing a wider tyre than was originally fitted on your tractor?

Better weight distribution to the ground
A wider tyre spreads the weight of the tractor over a larger surface area, which reduces the pressure exerted on each ground point. This limits soil compaction, which is essential for maintaining the fertility of your land and encouraging crop root development.
Working at a lower inflation pressure
A wider tyre can work with a lower inflation pressure if you don’t increase the load. This allows you to protect your soil better.
Increase in load capacity
A wider tyre contains a higher volume of air and therefore increases your load capacity. You can work with wider implements which improve your productivity.
Better traction in wet conditions
A larger contact area improves the tyre’s adherence to the ground. The full width of the lugs grip onto the ground simultaneously, which reduces the risk of slip, especially in wet conditions. This means that you can work more efficiently and reduce the amount of fuel required.

2. What are the dimensions of your rims?

What is the rim width?

The rim, the metal part which supports your tractor tyre, has very precise characteristics. One of the most important of these is its width. Expressed in inches, it corresponds to the distance between the two inner edges of the rim.

Why is this width important? It essentially determines the dimensions of the tyres that can be fitted. A rim that is too narrow cannot be fitted with a wide tyre, and vice versa.

Rim dimension diagram: diameter / width — 1. Height or diameter of the rim in inches, for a rim of 20 inches, 32 inches.
2. Width of the rim in inches, which is the number associated with the profile: W15, DW23, TW18…

Choice of tyres based on your rim:

What tyre sizes are possible if you keep the same rim?

Once you know the dimensions of your rim (width and height) you can consult the tyre manufacturers’ correspondence tables. These tables show the different tyre sizes compatible with your rim.

In general, it is possible to fit slightly wider tyres on the same rim. For example, if you have a W13×28 rim, you may be able to fit a 420/70 R28 tyre instead of a 380/85 R28 tyre. However, it is important to follow the tyre manufacturer recommendations to remain within the acceptable tolerance limits.

The particular case of NRO tyres

Certain more high-tech tyres such as VF tyres may have an NRO option (Narrow Rim Option) which makes it possible to fit wider tyres on rims that would have been too narrow for standard tyres.

The suppleness of the sidewalls and the design of the casing provide greater flexibility and provide the opportunity to opt for bigger, high-tech tyres without investing in wide rims.

The advantages of changing your rims

With wider rims you can fit wider tyres

Wider rims offer many benefits:

a better weight distribution,
a reduction in soil compaction,
better traction, etc.

By changing rims to accommodate wider tyres, you can optimise tractor performance levels.

Reduce the diameter of the rims with greater width

Reducing the diameter of the rim will enable a considerable increase in the volume of air contained in the tyre and either allow for a greater load capacity or limited inflation pressure, for soil preservation purposes.

For the same tyre height, a reduction in rim diameter with wider tyres allows you to increase the load

3. What load are you aiming for?

The load that you wish to carry with your tractor is a decisive element in your choice of tyres. Each tyre is effectively designed to bear a certain load, and if you wish to increase the load, you need to move on to a larger size of tyre.

Why is the load important?

The load index marked on your tyre sidewall indicates the maximum weight that a tyre can carry at a given speed. It contains a number followed by a letter.

For example:

For a 580 / 70 R38 tyre with a load index and speed rating 160 D / 157 E.
At a nominal pressure of 1.6 bar, it can carry 4065 kg (load index = 160) at 50 km/h (speed rating = D) or 3525 kg (load index = 157) at 70 km/h (speed rating = E).

This load index is important; if you overload your tyres you run the risk:

Of damaging the tyres: the sidewalls will distort excessively, the structure of the tyre is placed under too much strain and in extreme cases the tyre may burst.
Of reducing the tyre lifespan: too much load accelerates wear to the tyre lugs resulting in more frequent tyre replacement required.
Of reducing traction: an overloaded tyre needs to have high pressure to cope with the load; it will tend to sink further into the ground, which reduces its traction capacity and increases slip.
Of increasing fuel consumption: an overloaded tyre has more rolling resistance which leads to higher fuel consumption.

Choice of tyre depending on the load

Different load capacity depending on the tyre series

Identify the load represented by your front implements before choosing your tyres

It is essential to evaluate the load that each implement will represent in addition to the weight of your tractor. You can consult the manufacturer’s technical data sheets or carry out weighing operations if necessary to help with your evaluation.

VF tyres: why are they more suitable for heavy loads?

VF tyres are designed to carry heavy loads at a lower inflation pressure than standard tyres; their structure is much more resistant, which allows them to carry up to 40% more load. They allow for a better distribution of the load. They increase the tyre footprint on the ground, resulting in reduced pressure to the ground despite the load as well as limited soil compaction.

Pressure management to optimise the load:

Beyond the choice of tyre, it is necessary to adjust pressure based on the mounted implement and the load transfer.

Tyre pressure must be adapted to the actual load. A correspondence table supplied by the tyre manufacturer indicates the inflation pressure required based on load and speed.

The importance of correct pressure settings:

Insufficient pressure can lead to the distortion of the tyre and to premature wear.
Too much pressure increases rolling resistance, leads to much higher fuel consumption and reduces traction.

4. Which tyre for which tractor depending on size and power?

The choice of tyre size depends on several factors such as the size and the power of the machine, the type of farm and the kind of operations that you carry out.

Extra-wide 900 mm tyres, for example, are perfectly suited to field crop farms of over a hundred hectares. These tyres offer a high load capacity and better traction.

More standard 580 mm tyres can also be very efficient, but for medium-size farms, as obviously they cannot carry the same load.

Below is a selection of practical examples with the most suitable tyre fitting possibilities for specific levels of tractor power: