Conversion possibilities tractor tyres

Gear ratio, rolling circumference and lead

There is a close relationship between the dimensions of front and rear tyres on four-wheel drive tractors.

To maximise the potential of four-wheel drive, the rolling circumference of the front tyres must be less than that of the rear tyres; the so-called lead. Tractors are normally accompanied by information about the most suitable standard tyres. Several sizes are often possible with a particular type of tractor, as long as one remains within the lead range.

A lead of less than 0% and in excess of +6% have a negative impact on the tractor’s driving performance and steering (particularly in corners) and result in (excessive) wear in front and rear tyres and premature wear and tear in the transmission.

The ratio between the number of revolutions in the front and rear axle is very important in a mechanical four-wheel drive (this is determined by gears in the gearbox). This ratio is referred to as the gear ratio and, depending on the brand and type of tractor, is normally between 1.20 and 1.50.
 

Dynamic measurement of the gear ratio

1. Activate front-wheel drive and differential
2. Mark the front and rear tyre at the bottom or on the contact surface
3. Let rear wheel spin 10x (forwards)
4. Count number of revolutions in front wheel
5. Calculation: divide the front wheel revolutions by 10
For example: 13.5/10 = 1.35 The gear ratio!

Different width sizes on the same rim: convert from inches to mm

In some situations, you will want to use tyres with a different width size on an existing rim. This is possible, but can sometimes be difficult to convert tyre sizes from inches to mm. We are pleased to explain how this works.

An inch is equivalent to 25.4 mm. This means you can now calculate how a tyre size in inches can be converted into a tyre size in millimetres. This will not be an exact figure, but an estimate. For instance, a 12.4R20 tyres is 12.4 inches wide, which is the same as 315 mm. The tyre size that corresponds with this is the 320/85R20.

In this example, the number 320 is the width of the tyre in mm, 85 is the ratio between the height and width, and R stands for radial construction. The number 20 stands for the diameter of the rim in inches, which, of course, must be the same for both tyre sizes
 

Conversion tables

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

Like to know more?

Do you need advice? Please contact us, our experts will be pleased to assist you. Contact our experts at +31 (0)523 - 850 850 or by e-mail to info@heuver.com. You can also read all about agriculture on the agricultural knowledge-page.