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%. 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 serial number plate. The following pairs are possible:

     

    Gear pair

    Transmission ratio
     

    49/11

    4,455
     

    53/10

    5,300
     

    51/9

    5,667

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

    • 15,692

  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:

    • 1,446
    • 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:

IMPORTANT: If a different tire combination is selected, or new rear tires are selected with an SRI (speed/radius index) higher than the previous ones, 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 (7.0714)
 

I2 =

Transmission ratio of front axle
 

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 — 1,451

1,446

1,452 — 1,479

1,465

1,480 — 1,514

1,497

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

1,887 — 1,926

1,907

1,927 — 1,964

1,943

1,965 — 2,011

1,990

2,012 — 2,062

2,028

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

Sample calculation

The intention is to fit a 6430 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 output is 1.833.

  4. The rolling circumference of the tire on the rear wheel is 5238 mm (206.2 in.).

  5. The rolling circumference of the tire on the front wheel is 3932 mm (154.4 in.).

  6. Calculate the overall transmission ratio.

3932 * 7,0714 * 5,667 * 100

  

= 1,866

5238 * 15,692 * 102,75

  
   

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

OU12401,000177D-19-20070704