Do we need to replace all four tires?

[blacka4avant]

I have a 1994 S4 and the owners manual says that "All four wheels must always be fitted with radial ply tyres of the same size, type, make and tread pattern." They go on to say "...and the centre differential will be damaged by the constantly different wheel speeds." So I understand that, however, then it goes on to say "The use of tyres with varying degrees of wear will not adversely affect the four-wheel drive transmission." So what is going on? Is the "four wheel drive transmission" the same as the "centre differential" or not? Here are some dimensions for wear and tire sizes for comparison. Audi states min tread depth is 3mm New tread depth standard is 9mm So there could be as much as 6mm difference in tread depth, but for this example lets say the tires are only worn to 5mm when two are replaced. That means the difference is 4mm in tread depth or 8mm in diameter (.31") I have checked tirerack and most tires of the same size from different manufacturers have a diameter within .2" of each other and typically it is only .1" Can anyone shed some light on this apparent contradiction?

[Gary@Tirerack.com]

The tread depth of all four tires should be within 4/32nds of an inch of each other according to Audi.

[blacka4avant]

Okay so Audi says 4/32" which means a diameter difference of 8/32". So my question still remains, if new tires of different brands are .1" different why is that not recommended when in normal wear Audi says up to .25" is okay?

The reason for my persistence is that I just bought my S4 and it has almost new tires on it but the fronts and rears are different brands. I had not expected to need to buy new tires right away but of course do not want to damage the car.

Thanks for your help.

[Gary@Tirerack.com]

They should NOT be different brands! Match the same brand and model of tire.

[blacka4avant]

Gary

Thanks for your replies. Different brands (assuming the same diameter) should not be used because it will affect the handling (safety)?

Is that the only reason?

I appreciate your assistance.

[thedge]

I have a follow-on question. Last time I visited the dealer, the dealer stated that I needed 3 new tires since 1 was at 9/32" and the other three were at 6/32". From what was posted here earlier, it seems like all four tires are within the acceptable range from Audi AND within 4/32" of each other.

The reason for buying three was given due to tread depth. There were no uneven wear patterns at all.

So was this merely an attempt for the dealership to sell tires (at their markup) or do y'all tend to replace them when they reach 6/32"?

I'm trying to do right by this A6 but am just also trying not to be snookered.

[Reggie]

Yes the handling will be effected - It's OK for temp use to use different brands as long as the tread depth meets the 4/32 rule.

I have seen accidents caused by different brand use due to one set of tires loosing traction before the other set without the typical warning (noise or gradual slip).

[blacka4avant]

Thanks Reggie, I recognize that handling will be affected and that either end of the car may come loose before the other unexpectedly. As long as I will not ruin the center diff or other parts I am okay with this. Here in New Zealand most of the tyres are Chinese or Asian imports of lesser quality than tyres I would buy in the US and are not major brands (unless you spend a LOT of money for the Pirelli, Bridgestone, etc).

So both pairs of my tyres are strange name imports and both sets are probably equally as bad. The handling so far has at least been balanced with no loose end showing up in normal wet and dry driving conditions and a few spirited moments on the nice curvy roads we have here. Until I replace all four tyres with something good I will just be more careful and not drive at 10/10ths.

Yes, they spell tyres like that here, and no, the toilets do not flush backwards.

Cheers.