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On a 2004 VW R32 (which uses a Haldex AWD system), I have one damaged summer tire. All the tires have at least half of their usable tread depth left. At $200 ea, I'd like to replace only a pair of tires, which I'd mount on the front of the vehicle where the wear occurs more quickly.

A TireRack salesman said that some AWD vehicles have a tolerance of 2-3/32" tread difference between axles to avoid damage to the transfer case (his term, i'd call the component in question a center differential), and that I should check with VW before buying the new tires.

VW customer care gave the answer that as long as the two tires new tires are either both on the front or both on the rear, that it's OK. In other words, there's no tolerance limit. I find this somewhat hard to believe. A VW dealer I called gave the off-hand answer of 6/32".

Would having tires of different size on the front and rear axles put extra stress on the center differential of an all wheel drive car?

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I'll mark my own possible dupe, but there's some subtle (or not-so-subtle) differences between the conditions here –  mac Nov 27 '12 at 19:49
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I have always wondered if they will replace all four tires if you made a "roadhazard" replacement claim for one tire and the tread difference between the new and old exceeded 2/32". –  mikes Nov 27 '12 at 22:06
    
@mikes - I asked about that when the tire shop tried to sell me the road hazard insurance. They only replace the failed tire. I've heard that some shops have some kind of tire lathe that they can use to shave down new tires to match old ones, but I've never seen a shop that actually has such a thing. –  Johnny Mar 26 at 23:51
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2 Answers

up vote 3 down vote accepted

"It depends"... I've historically opted to replace all 4 just to be safe (on my AWD with 3 LSD beast, on the RWD open diff I replace 1 at a time if needed :-) ). In theory a small difference won't blow up the transfer case or the diffs (there's always at least a TINY difference anyways). Normal wear with a normal rotation obviously doesn't violate the limit, but 2 brand new tires with 2 at the wear bars supposedly does. Where exactly the line between "safe" and "unsafe" is, I have no idea. Actually, it's probably more of a gray area based on heat buildup...

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Anecdotally, another issue - a couple of years back I just got new fronts, and my Subaru got incredibly tail happy. Part of it would be down to the reduced grip at the back end, but part would be because of the extra torque being transmitted to the smaller radius rears. I replaced the rears a week later after ending up pointing the wrong way on a roundabout by accident! –  Rory Alsop Nov 28 '12 at 9:14
    
@RoryAlsop That probably happened because your mis-matched tire sizes caused the center diff to slip, overheat, and bind. That will quickly cause permanent damage to the viscous coupling, and will result in all kinds of wonky handling (unless you're on dirt, where it's just more awesome). For auto trans equipped Subarus the effect is similar, because the AWD system will engage the center diff clutch. –  Series8217 Aug 28 '13 at 21:10
    
For reference, Subaru's permitted difference in tire circumference is 1/4" between the smallest and largest tire. That ends up being between 1/32" to 2/32" difference in tread depth (tire radius). Not much. I don't have any specs from other manufacturers. –  Series8217 Aug 28 '13 at 21:13
    
Series - hadn't thought of that. I was embarrassed to have spun but assumed it was entirely me messing up. Reassuring to think it may have been exacerbated by a hardware issue:-) –  Rory Alsop Aug 28 '13 at 21:19
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Just as a note, awd systems I worked on in the past had a limit of 1 inch difference in circumference of any tire to avoid abnormal wear of the transfer case. And I did see several cases where the chain the case was already stretched with a bad jerking on take off, and new tires fixed the symptom although of course the underlying cause remained.

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