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:

1. On 6510 and 6610 tractors, ascertain transmission ratio of gear pair for differential. This information is displayed on the transmission type plate. The following figures are possible:

• 47/8
• 53/9

2. On 6510 and 6610 tractors, ascertain the transmission ratio of the front axle. This figure is displayed on the front axle type plate. The following ratios are possible:

• 13.29
• 15.86

3. Transmission ratio of gear pair for FWD axle (output). On 6510 and 6610 tractors, this information is displayed on the transmission type plate. On 6810, 6910 and 6910S tractors, it is displayed on the product information plate inside the l.h. service door. The following ratios are possible:

6810, 6910 and 6910S tractors

• 1.680 = gear pair A1
• 1.760 = gear pair A2

6510 and 6610 tractors

• 1.692 = gear pair F

• 1.725 = gear pair G
• 1.760 = gear pair H
• 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
• 1.990 = gear pair N
• 2.028 = gear pair O

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. 6510 and 6610 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.

6810, 6910 and 6910S 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 rolling circumference ratio of the rear tire to the front tire.

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Then, in the relevant table, look across the line (A1 or A2 for 6810, 6910 and 6910S, or F - O for 6510 and 6610) which refers to the gear pair fitted to the tractor and 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 range of values stated, either select a different tire combination or change the gear pair for the front wheel drive axle.

IMPORTANT: The largest front tires must not exceed an index radius of 620 mm (24.4 in.) on 6510 tractors, of 640 mm (25.2 in.) on 6610 tractors and of 670 mm (26.4 in.) on 6810, 6910 and 6910S 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.

Example

1. The intention is to fit a 6810 tractor with 18.4-38 / 14.9-28 tires, made by a certain manufacturer.

2. The transmission ratio of the gear pair at the front wheel drive axle is 1.760 (= gear pair A2).

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

• 18.4-38 = 5207 mm (205 in.)
• 14.9-28 = 4000 mm (157.5 in.).

The difference between the rolling circumferences is 1207 mm (47.5 in.), i.e. within the acceptable range.

The rolling circumferences ratio is:

Looking across the line from gear pair A2 we find the value 1.307, which is sufficiently close to the calculated value of 1.302. The front wheel lead of the desired tire combination is therefore 3%.

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