Hi. I've had my A4 for about a year now and it's my first AWD vehicle. I nailed a huge pothole today and popped one of my tires. They're getting pretty bald anyways, and I was planning on getting new rims and rubber in about two months. Now, I'm not sure what to do. I'm gonna take in the tire tomorrow to see if it can be repaired. If it can't, I'd like to but a used tire to put on there for the next few months, but I had a friend (subaru owner) who advised me against this, saying that it is critical with AWD cars to have very closely matched tires (matched as far as tire diamter). I did a little research on the web and found sites that contradicted each other. Some of them were saying that even slight differences in tire diameter, even as a result of uneven tread wear, can actually damage the differential . Some of the sites said that in a true AWD car, and Audis obviously qualify, that this would not be a major concern. I was surprised by this being a potential problem. For one thing, the factory spare, which I currently have on there, is not a full size spare. Basically, my dilemma is that I don't want to get a new set of tire for the factory wheels, as I was planning on changing them in a couple of months when I would have the cash. But, I have to do something about that tire, and I certainly don't want to damage my drive train by mismatching the tires. I'm pretty sure that I could find a used tire that would be close to the tread depth, but according to one of the sites, Audi insists on all the tires being with 4/32" of each other. I'm not sure I could find a tire that is THAT close. If I find a used tire that has more of a difference, will it actually do any damage over a couple of months? Thanks.

 

Not sure what the correct answer is on this one, but after a recent post on differentials, here are my thoughts. With one tire a different size, the smaller tire will effectively be turning faster than the other. It's basically like you are in a perpetual turn with a very large radius. Here is an explaination of what happens with an open differential in a turn....

"When you're driving around a turn, this all works together. The inside tire must turn slower than the outside. With constant throttle the housing speed remains fixed, but the spider gears turn very slowly which allows the wheel on the inside of the turn to rotate a little slower than it normally would and the wheel on the outside of the turn to rotate faster than it normally would. The housing rotational speed is always the average of the two sides. Great for going around turns at slow speeds. "

I'm no expert, but it seems to me the different size tire will just cause the differential to put in some overtime. It will do what it's designed to do....allow one wheel to turn at a different speed than the other. I don't know what kind of pressure is introduced to the differential's gears when this is happening, but it's probably not good to have the constant pressure for long periods of time. That said, I don't think LIMITED amounts of driving for a month or so will cause the differential to go bad, but then again, is it worth the chance.

Again, I'm no expert in differentials or cars in general. I'm not sure this helps any, just found this question interesting and wanted to start the discussion.

 

You will need to replace all 4 tires. The quattro system is designed to work w/ all 4 tires within 4/32". If you drive w/ one tire that is different for a longer period of time, it will eventually destroy that axle. The "donut" spare is designed to be used for a short period of time, 100 miles.
Jukeit has a nice post on what happens in the last post.

 

But how long is too long? Can he go a couple of months without damaging the differential? I'm thinking it would be ok if he finds a full size tire that is somewhat close in treadwear and driving is limited, but I would like to hear from anyone who knows for sure.

I'm curious to know specifically what the damage would be. Not just "it will ruin it", but an explaination of exactly why and what parts of the differential get ruined.

 

I left a message w/ a friend of mine, service manager at Audi, to see exactly what happens, i'll let you know.

 

I've been calling around a bit and have been getting varying advice even from different Audi dealerships. One guy at one Audi dealership said that he's been working there for 10 years and never heard of anyone having a problem damaging their differential from different tire size. He said that he is "supposed" to tell me to change all 4, but he said that between the two of us, he wouldn't hesitate to put on one different tire. The whole thing seems a little unreasonable to me to be honest. I mean, I wouldn't be surprised to find out that between different amounts of tread wear and a tire or two being underinflated, that many of us Audi owner's have driven their quattro for extended periods of time with more of a difference than a few 32nds. I'm curious to see what Kellya4's friend at the dealership has to say about it. Thanks for the input guys.

 

Good point Ostojo. Since I've been on the various forums, I can't ever recall seeing a problem with the differentials. Thanks for making the call Kelly.

 

Put a used tire on as a replacement, you will be fine... absolutely no worries. Make sure the tire is the same size though obviously... like replace a 205/55/16 with another... but don't worry about tread... you WILL be fine. Oh, and if you can, try and get tire diameters... not all 205/55/16's have the same diameter... they are all fairly close... within .3 of an inch I believe... but you might want to look into that. It wouldn't be a big deal though, either.



EDIT: if you're in a place with snow... you might want to consider purchasing 4 snow tires... and driving them until you pick up the summer wheels/tires... that way you are good to go all year long.

 

I bought my audi about 2 months ago and I notice that the tires have different brands. They're all not the same. Reading this got me worried. I should look into new tires soon.