IMPORTANT: Stay with tire dimensions as identified in Group A OR Group B. Tractor approval- / homologation

rules do not permit switching between groups.

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%. Positive front wheel lead should never be less than 1% and never greater than 7%, otherwise the tires will become deformed or suffer undue wear. To ascertain the correct ratio when changing tires, proceed as follows:

Ascertain tractor data:

6520 and 6620 tractors

6820, 6920 and 6920S tractors

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

6520 and 6620 tractors

• 15.857

6820, 6920 and 6920S tractors

• 13.227
• 13.255
• 13.286

3. Transmission ratio of gear pair for FWD axle (output). On 6520 and 6620 tractors, this information is displayed on the transmission type plate. On 6820, 6920 and 6920S tractors, it is displayed on the product information plate inside the l.h. side of the radiator. The following ratios are possible:

6520 and 6620 tractors

• 1.446
• 1.465
• 1.497
• 1,532
• 1.564
• 1.597
• 1,625
• 1.658
• 1.692
• 1.725
• 1.760
• 1.795
• 1.833

OU12401,00005A5 -19-01JUL02-1/3

• 1.870
• 1.907
• 1.943
• 1.990
• 2.028

6820, 6920 and 6920S tractors

• 1.455
• 1.485
• 1.515
• 1.545
• 1.680
• 1.760
• 1.760
• 1.800

Ascertain tire data:

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.

IMPORTANT: The largest front tires must not exceed an index radius of 620 mm (24.4 in.) on 6520 tractors, of 640 mm (25.2 in.) on 6620 tractors and of 670 mm (26.4 in.) on 6820, 6920 and 6920S tractors.

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

6520 and 6620 tractors

The difference in rolling circumferences between the front and rear tires must be between 1035 mm (40.7 in.) and 1756 mm (69.1 in.). However, a difference of between 1106 mm (43.5 in.) and 1684 mm (66.3 in.) is desirable.

6820, 6920 and 6920S tractors

The difference in rolling circumferences between the front and rear tires must be between 782 mm (30.8 in.) and 1505 mm (59.3 in.). However, a difference of between 855 mm (33.7 in.) and 1433 mm (56.4 in.) is desirable.

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 (7.0714 for 6520 and 6620 tractors, and 6.40 for 6820, 6920 and 6920S tractors)
	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.

6520 and 6620 tractors

OU12401,00005A5 -19-01JUL02-2/3

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)
1.428 - 1.446	1.446
1.447 - 1.478	1.465
1.479 - 1.512	1.497
1.513 - 1.544	1.532
1.545 - 1.576	1.564
1.577 - 1.617	1.597
1.618 - 1.637	1.625
1.638 - 1.669	1.658
1.670 - 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
1.883 - 1.918	1.907
1.919 - 1.964	1.943
1.965 - 2.002	1.990
2.003 - 2.062	2.028

 

6820, 6920 and 6920S tractors

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)
1.437 - 1.466	1.455
1.467 - 1.495	1.485
1.496 - 1.525	1.515
1.526 - 1.567	1.545
1.659 - 1.698	1.680
1.699 - 1.737	1.720
1.738 - 1.777	1.760
1.778 - 1.822	1.800

 

Example

The intention is to fit a 6520 tractor with 650/65R38 and 540/65R28 tires, made by a certain manufacturer.

1.

The transmission ratio of the differential gear pair is 4.700.

2.

The transmission ratio of the front axle is 15.857.

3.

The transmission ratio of the gear pair for the front wheel drive axle is 1.597.

4.

The rolling circumference of the tire on the rear wheel is 5458 mm (214.9 in.).

5.

The rolling circumference of the tire on the front wheel is 4230 mm (166.5 in.).

6.

The difference between the rolling circumferences of the tires on the rear/front wheels is 1228 mm (48.3 in.), i.e. within the acceptable range.

7.

Calculate the overall transmission ratio.

4230 * 7.0714 * 4,7 * 100
	= 1.581
5458 * 15.857 * 102.75

 

The gear pair that is required must have a ratio of 1.597.

OU12401,00005A5 -19-01JUL02-3/3