audiblack73

tigerwillow1

There's a statement about it in the "Driving with your quattro" section of the owners manual. It's on page 296 of my 2008 manual. The summary is that all 4 tires should be the same size and tread depth. But just to muddy the waters at least a bit, the "New tires" section on page 361 it says tires should be replaced at least in pairs.

bspurloc

It's not true no matter what anything or anyone says.
If you were using the car as a rally racers maybe, but driving normal absolutely not.
You always have the choice to say NO I only want 2 tires. The only reason shops say it is to make more money.
I'll also note it is a bit absurd as the QUATTRO AWD system is pretty insane, you are not going to lose traction with this system because 2 wheels have less tread.

ALSO to note, your rear tires are worn down worse than your fronts, so already you have been violating this rule that all 4 must be the same.

dreadlocks

a differential allows two or more (on AWD) wheels to rotate at different speeds and still be powered.. w/Quattro normally when driving in a straight line down the highway all 4 wheels will be in equilibrium and the differentials will basically be locked and transferring power to all 4 corners in whatever split they were designed for.. rear diff will be splitting power evenly between both rear wheels and the gears wont be turning on the planetary gear, front diff will be splitting power evenly between front two wheels in the same way and the center diff will be splitting power 40:60 in favor of the rear in much the same way.

The differentials will only kick in and do their thing to allow the wheels to spin at different speeds when your turning or if a wheel starts loosing traction otherwise, duty load on them will be partial for normal day to day driving.

With one mismatched size tire you'll have one differential engaged constantly on one end axle, then the center diff will be engaged constantly because that end is out of balance.. so two diffs will now be at 100% duty cycle.

With two tires mismatching the other two the end differential will be in equilibrium driving in a straight line but the center diff will be out of balance and sending more/less power to that side at a 100% duty cycle..

Is this bad overall for general traction? nah, thats the point of a mechanical awd system is that it can adapt to these conditions, you probably wont even notice.. is it bad for the drivetrain? oh **** yeah it is when your in motion your basically never giving at least one differential a break. When it starts whining because excess friction causes heat and wears down the lubrication you'll know its because you ignored the manufacturer's recommendations and went with the advice of some internet guy, but good news is you catch it quick enough the wine wont get worse and it'll still last a good while longer with fresh fluids and correct size tires.. but the whine most certainly wont go away until you have a differential suicide.

Your supposed to replace all your tires together, if you dont like it you dont have to do it.. but there is a reason why you should, they are not making up crap to sell you more tires.. I had a friend try 3 different discount tire shops around here and all refused his business until he was willing to buy 4, yeah totally after it for the money since he didnt give any of the shops a dime in the end.. they always have the choice to tell you to take your business elsewhere and if you went to a decent tire shop the'd do the same.

chingwodapigu

You should rotate your tires more often so they wear more evenly.

bspurloc

tire shops claim its a safety issue because everyone is racing on wet roads etc.
as for 2 new tires against 2 75% tires causing differential issues I am going to need to see that in a video showing the differential actually is that sensitive rather than trust the word of someone on the internet especially since people set on an opinion tend to exaggerate to push their point.
I am kinda thinking it would be a bad design to be that sensitive there would be lots of differentials being worn out due to people dont notice their rears or fronts are badly worn until some shop tells them.

dreadlocks

hey whattya gonna do? pick between two idiots arguing on the internet or go with whats clearly written in your manual by the manufacturer.. this guy here wants proof he's abusing his drivetrain, but dont trust the manual he's not gonna believe anyone with any logic or facts behind em.

If you cant afford to maintain an Audi properly, next time might I suggest buying a Toyota instead.

tigerwillow1

This issue will probably get debated forever unless Audi makes a clearer statement about it, because what they say is open to some interpretation. My urS4 manual (car has torsen center diff) makes these statements:

1. For safety reasons, tires should be replaced in pairs and not individually. The tires with the deepest tread should always be mounted on the front wheels.
2. All 4 wheels must have tires of the same size, construction, and tread type.
3. Different tires on front and rear may impair vehicle control and damage the AWD because of different wheel speeds.
4. The proper function of the system is not affected by unevenly worn tires.

Point 1 says replace in pairs. Not much disagreement over that, although if a fairly new tire has an unrepairable failure I wouldn't throw the other almost new tire away. Point 1 also acknowledges there are normal tread depth differences.
Point 2 says tread tread "type", not "pattern".
Point 3 doesn't define exactly what "different" applies to.
Point 4 seems to at least somewhat contradict some of the other statements.

This will get resolved only if Audi defines "tread type", and the maximum allowable tread depth difference.

lentiman

I think we can all agree that it's better to rotate your tires every 6-12 months to avoid uneven wear. The real kicker is when you have one tire blow out and is unrepairable so it has to be replaced, where do you draw the line on all 4 tires? It's a $70K car, if you don't want to maintain it you should have considered other options at time of purchase, right? It would be nice to see how many 32nd's of tread difference matter in the real world. Those numbers probably exist somewhere but I doubt Audi or any other AWD manufacturer will ever release them to the public. It would also be nice if there was a display setting that listed the amount of differential engagement. I wonder if there's a sensor at the diffs...

yosie16

dealer told me last week my rear tires were below ideal level, recommended I replace the rears only at this time. I did just that. said they would rotate them up when i brought in to buy fronts (not needed at this time).