I need to replace one tyre on the rear axle of my '15 S3 Quattro. How important is it to keep the tread pattern the same for tyres on the same axle? I've been told different treads on the same axle can affect the Quattor system.
Really you want to keep the tread patterns and more importantly the tread depth the same across all four - but at worst keep it the same across the axle.
In the case of the pattern It's not really down the tread pattern itself needing to be the same so much as it is about keeping the grip profile the same across the tires and same brand, same model is the easiest way to do that.
For the depth this is to keep the rolling radius as close as possible so as to avoid excess wear on the differentials.