Tire combinations

The size ratio of the front wheels to the rear ones is precisely determined in order to produce a positive front wheel lead of between 1.5 and 4%. To ascertain the correct ratio when changing tires, proceed as follows:

Ascertain tractor data:

1. Transmission ratio of the differential gear pair. This value can be found on the transmission serial number plate. The following pair is possible:

   	Gear pair	Transmission ratio
   	53/13	4,077

2. Transmission ratio of front axle. This value is displayed on the front axle type plate. The following ratio is possible:

   • 11,625
   
   

3. Transmission ratio of gear pair for front-wheel drive axle output. This value can be found on the transmission serial number plate. The following ratios are possible:

   • 1,692
   • 1,725
   • 1,760
   • 1,795
   • 1,833
   • 1,870
   
   

Ascertain tire data:

This information must be ascertained from the tire manufacturer's manual.

1. Select tires with suitable load-bearing capability.

2. Select tires appropriate to the tractor's top speed.

3. From the manual, ascertain the rolling circumference of the tire desired for the rear wheel.

4. From the manual, ascertain the rolling circumference of the tire desired for the front wheel.

Ascertain the following data:

1. Calculate the difference between the rolling circumferences of the tires on the front and rear wheels.

   If a new pair of tires is selected, the basic informator (BIF, if equipped), the Basic Control Unit and the ground speed sensor (radar) must be recalibrated. See your John Deere dealer.

   The difference in rolling circumferences between the front and rear tires must be between 945 mm (37.2 in.) and 1618 mm (63.7 in.).

2. Calculate the overall transmission ratio using the following formula:

		A1 * I1 * I3 * 100
	I =
		A2 * I2 * 102.75
	A1 =	Rolling circumference of front tires
	A2 =	Rolling circumference of rear tires
	I1 =	Rear axle ratio (6.857)
	I2 =	Front axle ratio
	I3 =	Transmission ratio of the differential gear pair

Ascertain the gear pair that is appropriate for the overall transmission ratio.

Calculated transmission ratio	Transmission ratio of the required gear pair
(corresponds to a pos. front wheel lead of 1.5 - 4.0%)	(corresponds to desired average value of pos. front wheel lead)
1,671 — 1,703	1,692
1,704 — 1,737	1,725
1,738 — 1,772	1,760
1,773 — 1,810	1,795
1,811 — 1,846	1,833
1,847 — 1,882	1,870

Sample calculation

The intention is to fit a 5620 tractor with 16.9-30 and 12.4-24 tires, made by a certain manufacturer.

1. The transmission ratio of the differential gear pair is 4.077.

2. The transmission ratio of the front axle is 11.625.

3. The transmission ratio of the gear pair for the front wheel drive axle is 1.833.

4. The rolling circumference of the tire on the rear wheel is 4418 mm (173.9 in.).

5. The rolling circumference of the tire on the front wheel is 3489 mm (137.4 in.).

6. The difference between the rolling circumferences of the tires on the rear/front wheels is 974 mm (38.3 in.), i.e. within the acceptable range.

7. Calculate the overall transmission ratio.

3489 * 6,857* 4,077* 100
	= 1,848
4418 * 11,625 * 102,75

The gear pair that is required must have a ratio of 1.870.