Tractor tyre conversion: sizes, rolling circumference and lead ratio

What is tractor tyre conversion?

Tractor tyre conversion means selecting an alternative tyre size while keeping the correct relationship between the front and rear tyres. This is especially important for four-wheel drive tractors, where the front and rear axles are mechanically linked.

When converting tractor tyres, always check:

• the front tyre size;
• the rear tyre size;
• the rolling circumference of both tyres;
• the tractor’s transmission ratio;
• the front axle lead;
• the rim diameter and rim suitability.

Several tyre sizes may be possible for one tractor type, but only if the tyre combination remains within the correct lead ratio range.

Gear ratio, rolling circumference and front axle lead

To make optimal use of four-wheel drive, the rolling circumference of the front tyres must be smaller than that of the rear tyres. The relationship between the front and rear rolling circumference is known as the front axle lead or lead ratio.

A lead ratio below 0% or above +6% can negatively affect driving performance and steering behaviour, especially when cornering. It can also cause excessive wear of the tyres and premature wear of drivetrain and transmission components.

With mechanical four-wheel drive tractors, the ratio between the number of revolutions of the front and rear axles is fixed. This ratio is determined by the gearbox gearing and usually lies between 1.20 and 1.50, depending on the tractor make and model.

An incorrect lead ratio can also contribute to soil compaction, because the tyres no longer work together efficiently.

Front axle lead formula

The front axle lead can be calculated with the following formula:

Front axle lead (%) = ((front tyre rolling circumference × transmission ratio) / rear tyre rolling circumference − 1) × 100

This calculation shows whether the selected front and rear tractor tyres are suitable for use on a four-wheel drive tractor.

In general, the lead ratio should remain between 0% and +6%. Always check the specifications of the tractor manufacturer before fitting an alternative tyre size.

How to measure the transmission ratio

If the exact transmission ratio is not available in the tractor manual, it can be measured manually.

1. Engage four-wheel drive and lock the differential.
2. Mark the front and rear tyre at the contact point with the ground.
3. Rotate the rear wheel forward exactly 10 full revolutions.
4. Count the number of revolutions made by the front wheel.
5. Divide the front wheel revolutions by 10.

Example:

If the front wheel rotates 13.5 times while the rear wheel rotates 10 times, the transmission ratio is:

13.5 / 10 = 1.35

This means the transmission ratio is 1.35.

Why is rolling circumference important?

Rolling circumference is the distance a tyre covers during one full revolution. When changing tractor tyre sizes, the rolling circumference is one of the most important values to check.

If the rolling circumference of the front and rear tyres does not match the tractor’s transmission ratio, the tyres can work against each other. This may result in:

• higher fuel consumption;
• excessive tyre wear;
• drivetrain and transmission wear;
• difficult engagement or disengagement of four-wheel drive;
• reduced traction;
• poor steering behaviour;
• slip damage in the soil;
• increased soil compaction.

The exact mechanical transmission ratio of most tractors can be found in the operator’s manual. The rolling circumference of agricultural tyres can be found in our agricultural tyre documentation and in our webshop.

Different tyre widths on the same rim: converting inches to millimetres

In some situations, you may want to fit tyres with a different width on an existing rim. This is possible in many cases, but converting tractor tyre sizes from inches to millimetres can be complex.

One inch equals 25.4 mm. This makes it possible to convert an inch tyre size to an approximate metric tyre size. In most cases, the result is not an exact number but a close equivalent.

For example, a 12.4R20 tyre is 12.4 inches wide. This equals approximately 315 mm. The corresponding metric tyre size is 320/85R20.

In this example:

• 320 indicates the tyre width in millimetres;
• 85 indicates the aspect ratio between tyre height and width;
• R stands for radial construction;
• 20 indicates the rim diameter in inches.

The rim diameter must always be the same for both tyre sizes.

Tractor tyre conversion table

You can use the table on the following page to convert tyre sizes in inches to sizes in mm.

Frequently asked questions about tractor tyre conversion

What is front axle lead on a tractor?

Front axle lead is the percentage difference between the rolling movement of the front and rear tyres on a four-wheel drive tractor. A correct front axle lead helps maintain traction, steering stability and drivetrain efficiency.

What is the correct lead ratio for tractor tyres?

In general, the front axle lead should remain between 0% and +6%. Always check the tractor manufacturer’s specifications, as the recommended range may differ per tractor make and model.

Why is rolling circumference important when changing tractor tyres?

Rolling circumference determines how far a tyre travels per revolution. If the front and rear tyre rolling circumferences do not match the tractor’s transmission ratio, it can cause tyre wear, drivetrain stress, reduced traction and poor steering behaviour.

Can I fit wider tyres on the same tractor rim?

In some cases, yes. However, the replacement tyre must match the rim size and must remain within the correct rolling circumference and front axle lead range. Always check the technical specifications before changing tyre sizes.

How do I convert tractor tyre sizes from inches to millimetres?

Multiply the inch width by 25.4 to convert it to millimetres. For example, 12.4 inches equals approximately 315 mm. The closest corresponding metric tyre size is 320/85R20.

Where can I find the rolling circumference of a tractor tyre?

The rolling circumference can usually be found in the tyre manufacturer’s technical data, in agricultural tyre documentation or in the Heuver webshop.