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 wheels, proceed as follows:

1. Gear pair on differential. The gear pair is displayed on the transmission type plate. The following pair is possible:

• 53/9

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

• 15,89
• 17,54 (for high clearance tractors only)

3. Transmission ratio of gear pair for front wheel drive axle. This figure is displayed on the transmission type plate. The following ratios are possible:

• 1,795 = gear pair I
• 1,833 = gear pair J
• 1,870 = gear pair K
• 1,907 = gear pair L
• 1,943 = gear pair M

These data 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. Ascertain the rolling circumference of the desired rear tires from the manual.

4. Ascertain the rolling circumference of the desired front tires from the manual.

1. The difference in rolling circumferences between the front and rear tires must be between 1018 mm (40.1 in.) and 1546 mm (60.9 in.).

2. Check that the index radius (to ISO Standard 4251) of the rear wheel lies within the relevant range (see also tables in "Specifications"):

• not more than 820 mm (32.3 in.) with 53/9 differential, e.g. 18.4R38
• not more than 874 mm (34.4 in.) with 53/9 differential, e.g. 13.6R46 (for high clearance tractors only)

3. Calculate the rolling circumference ratio of the rear tire to the front tire.

4. Using the data on the differential gear pair and the front axle transmission ratio, ascertain which of the following tables applies.

In the line (I - M) with the gear pair that applies to the tractor, find the value that corresponds to the calculated rolling circumference ratio.

The column gives the front wheel lead that may be achieved with the tire combination.

If the calculated ratio does not lie within the stated value range, either a different tire combination must be selected or else the gear pair for the front wheel drive axle must be changed.

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IMPORTANT: 6405 tractors: The front tires must not exceed an index radius of 540 mm (21 in.) - 780 mm (31 in.) on High Clearance tractors.

6605 tractors: The front tires must not exceed an index radius of 590 mm (23 in.) - 810 mm (32 in.) on High Clearance tractors.

If a new pair of tires is selected, the instrument unit must be reset. See your John Deere dealer.

The intention is to fit a tractor with 18.4-34 / 14.9-24 tires, made by a certain manufacturer.

The differential gear pair is 53/9.

The transmission ratio of the front axle is 15,89.

The transmission ratio of the gear pair for the front wheel drive axle is 1,425 (= gear pair I).

According to the tire manufacturer's manual, the desired tires have the following rolling circumferences:

• 18.4-34 = 5003 mm (197 in.)
• 14.9-24 = 3795 mm (149.4 in.).

The difference between the rolling circumferences is therefore 1208 mm (47.6 in.), i.e. within the permitted range .

The index radius (to ISO Standard) of the rear tire is 770 mm (30.3 in.). The tire is therefore compatible with the differential.

The ratio of rolling circumferences is:

In this example, the correct table is Table 1 (53/9 differential and 15,89 transmission ratio at front axle). The line for gear pair L includes the figure 1,321, which is sufficiently close to the calculated value of 1,318. The front wheel lead of the desired tire combination is therefore 4%.

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