Tire Combinations (up to 40 km/h; 25 mph)

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%, otherwise the tires will become deformed or suffer undue wear. To ascertain the correct ratio when changing tires, proceed as follows:

Ascertain tractor data:

1. Transmission ratio of the differential gear pair. This information is displayed on the transmission type plate. The following pairs are possible:

  Gear pair Transmission ratio
  49/11 4.455
  47/10 4.700
  53/10 5.300
  51/9 5.667
  47/8 5.875

 

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

  • 13.125
  • 15.692
  • 19.000

3. Transmission ratio of gear pair for FWD axle (output). This figure can be found on the transmission serial number plate. The following ratios are possible:

  • 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
  • 1.870
  • 1.907
  • 1.943
  • 1.990
  • 2.028

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.

IMPORTANT: The largest front tires on 6020, 6120 and 6220 tractors must not exceed an index radius of 610 mm (24.0 in.). The largest front tires on 6320, 6420 and 6420S tractors must not exceed an index radius of 620 mm (24.4 in.).

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.

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.).

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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)
  I2 = Front axle ratio
  I3 = Transmission ratio of the differential gear pair

 

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

 

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

Sample calculation

The intention is to fit a 6420 tractor with 600/65R38 and 540/65R24 tires, made by a certain manufacturer.

1. The transmission ratio of the differential gear pair is 5.667.

2. The transmission ratio of the front axle is 15.692.

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 5263 mm (207.2 in.).

5. The rolling circumference of the tire on the front wheel is 3923 mm (154.4 in.).

6. The difference between the rolling circumferences of the tires on the rear/front wheels is 1340 mm (52.8 in.), i.e. within the acceptable range.

7. Calculate the overall transmission ratio.

3923 * 7.0714 * 5.667 * 100  
= 1.852
5263 * 15.692 * 102.75  
   

 

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

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