Tire Tolerance Logic

The tire tolerance logic detects different tread circumferences on the front and rear axles. This occurs when:

• Mixed tires are used.
• Space saving spare tire is installed.
• Tires are used that have been worn down to different levels.

Normally, tire circumference deviations result in drivetrain torque bias (unwanted variations). The tire circumference can fluctuate up to 1 percent or more as a result of mixed tires or wear. The tire tolerance logic decides depending on the driver's command and driving situation whether the slip is to occur in the transfer case clutch or at the contact area between tire and road. If the slip is permitted in the transfer case clutch, the locking pressure set by the pre-control is reduced in order to keep the work loss low. In the driving dynamic control situation, the clutch is locked slightly more than normal, the four wheel drive is always guaranteed when required. For maximum xDrive performance, tires (and wheels) of the same diameter should be installed on the vehicle.