My '99 A6 came with a "space-saver" spare which is much smaller then the stock tires, I only ran it for three days, but did not have any trouble, in fact it drove very well. There are always going to be different tread wear or tire pressures anyway, keeping within 2/32 seems difficult. I'm not sure how the quattro system handles different tire speeds, but every time you take a corner you've got tires spinning at different rates, according to other posts the quattro system is purely mechanical so must be able to compensate for this somehow.

 

Different sized tires rotate at different RPMS when in a vehicle -- if they are on the same axle or connected to the same drive system as in a the quattro the difference is speed has to handled someplace. It is usually in a differential. The difference in speed results in heat and abnormal wear on the components.

The differentials have to be there for turning -outer tire going faster than inner tire- but it is only active some of the time... Ever feel your Quattro in a bind when turning? Think what would happen if it was in that bind all the time.... UGH - lots of stress on the system..

Bottom line -- The damn tire need to be close to the same size or you'll screw up you vehicle...

 

why dont you just fix the flat??