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 figure is displayed on the front axle serial number plate. The following ratio is possible:

   • 11,625
   
   

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

   • 1,532
   • 1,564
   • 1,597
   • 1,625
   • 1,658
   • 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:

IMPORTANT: If a different tire combination is selected, or new rear tires are selected with an SRI (speed/radius index) higher than the previous one, the tractor's electronics must be recalibrated by your John Deere dealer.

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 =	Transmission ratio of front axle
	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)
1,515 — 1,548	1,532
1,549 — 1,580	1,564
1,581 — 1,614	1,597
1,615 — 1,645	1,625
1,646 — 1,672	1,658
1,673 — 1,711	1,692
1,712 — 1,739	1,725
1,740 — 1,779	1,760
1,780 — 1,812	1,795
1,813 — 1,853	1,833
1,854 — 1,886	1,870

Example

The intention is to fit a 5080R tractor with 16.9R30 and 12.4R24 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 1.658.

4. The rolling circumference of the tire on the rear wheel is 4395 mm (173.0 in.).

5. The rolling circumference of the tire on the front wheel is 3420 mm (134.6 in.).

6. Calculate the overall transmission ratio.

3420 * 6,857* 4,077* 100
	= 1,821
4395 * 11,625 * 102,75

The gear pair that is required must have a ratio of 1.833.