Jet Jockey/A4 Pilot

I had the chance to talk to the head technical person at my dealership today. According to him there is no actual written technical info on this from Audi. He as seen cars over the years with mismatched wheels and tires with no ill affect to the Quattro drive or TorSen. Now on the Porsche side, they will not allow you to change a tire or a pair of tires if they are more than 60% worn. In other words, you can not change one or only two tires if the remaining tires have less than 40% of thread wear left on them.

He said that would also make sense to use as a rule of thumb for Audi but it would be a safety related reason and not a possible mechanical failure reason.

I hope this makes some sense.

SOASOAS

Just to keep the math simple, let's assume a tread depth new of 10/32 (pretty common for performance tires). At 60% tread wear, the rolling radius of the wheel has been reduced by 6/32 (OK 3/16) of an inch. While I suspect that the majority of folks here are pretty good at keeping our tires at the proper inflation, I would be willing to bet that one could find a fair number of quattro cars with better than a 3/16ths difference based on air pressure alone. While I can see that Audi (or Porsche or whoever) may not want to compound the air pressure deltas with the tread deltas, I would expect to have heard of quite a few failures by now if the corner to corner tolerances were really that tight. Of course, I'm not suggesting that one could go to the extremes that RWD cars can afford with tire size differences - it just seems that there has to be a bit of cushion in the design of the system.

JMSIV

With only 1% difference, I feel pretty confident it will be fine. I wonder what running different pressures does; I mean, I have always run the recommended 38 front 35 rear. I would think that would change the effective radius, but maybe not. Anyway, thanks for the input.