Mismatched tires on tandem drive units will cause the following conditions:

1. Match the tires to within 3 mm (0.125 in) of the same rolling radius and 19 mm (0.75 in) of the same rolling circumference.

Notice: The four largest tires should never be installed on one driving axle or the four smallest tires on the other driving axle. Such tire mounting will cause an inter-axle bind and unusually high axle lubricant temperatures that result in premature lubricant breakdown and possible costly axle damage.

2. Ensure that the total tire circumference of one driving axle matches the total tire circumference of the other driving axle as nearly as possible. This will result in higher tire mileage. This will also maintain satisfactory temperatures of tandem axle lubricants which lengthen the drive unit service.
3. Park the vehicle on a level floor, carrying an evenly rated capacity load. Verify that all of the tires are the same size. Measure new tires in order to be sure that the tires will be correctly matched.
4. Inflate all tires to the same recommended pressure.



5. Measure the rolling circumference of each tire. Use a steel tape after you have inflated the tires and placed the tires on their rims, but before you have installed the tires on a vehicle. This is the most accurate method. This procedure allows for any irregularities in wear.



6. Check the tires already installed on the vehicle. Use one of the following devices:

•	A large square

•	A string gauge (1)

•	A large caliper

•	A matching stick

•	A wooden straight edge

•	Ensure that the device is long enough to lie across the treads of all four tires.

7. Mark the size on each tire with chalk. Arrange the tires in order of size, largest to smallest.

Installation Procedure

1. Install the two largest tires on one side of one axle.
2. Install the two smallest tires on the opposite side of the same axle.
3. Install four other tires on each of the other axles as described in steps 1 and 2.
4. Check the tires using a string gauge (1).
5. Inspect the rear axle lubricant temperature readings.

•	Run the vehicle for accurate readings.

•	Vary the tire air pressure, within the tire manufacturer's recommended range, in order to ensure that the lubricant temperature of both axles is within 17°C (30°F) of each other and not in excess of 104°C (220°F). This will result in uniform tire loading and will assure good tire life.