Permitted Differences In Tread Depth: Notes

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For all four wheels use only tires of the same construction type, and tread.
Replacing one individual tire is always possible. However, Audi recommends at the very least that the tires are replaced on both sides of the axle.
With sport differential on the rear axle, Audi recommends always replacing the tires on both sides of the rear axle.

NOTE:

The technical background of the limitations are thermal pressure for the transmission oil due to a large difference in the rolling circumference of the tires.