fact vs. fiction? wheels must be same diameter on AWD

[rallysam]

I'm sure this is a FAQ, but I couldn't find anything in the archives.

I have often heard you can have differential problems on our cars if you run tires that aren't the exact same diameter or drive too far on a donut. I understand completely, but I've never heard a common-sense answer to how much of an issue this really is.

1) Is this only a problem for viscous LSD's? Or is it a problem on any Subaru AWD?


2) If my tires are worn down but I replace one tire with a fresh tread, will this cause me problems? How closely matched does the tread-depth have to be on all 4 tires to avoid said problems? I am not looking for the conservative answer that Subaru puts in the ownders manuals (to be safe and avoid being sued). I am just interested in the common sense answer.


3) How far can I really drive on a donut? And what happens to me when I ignore that?

Ideally, I want to be able to drive 300 miles home from the middle of nowhere on a donut - but I know a full-sized spare is probably necessary. In that case, my full-sized spare will have the problem I described in Question 2.


Thanks!
Sam
1990 Legacy L Wagon AWD

[DLC]

With a normal 2WD car, you will have problems if you have different sized tires on the left and right drive wheels. Now take that to the extreme, where you have not one, but three differentials. This is why you need to have the tires as close to the same size as possible.

I blew out a tire and ran my spare for no more than 5 blocks. In that time, i could actually hear the differentials complaining as they had to work especially hard to compensate for the smaller size. I know they ship it out with that size, but i've never come upon any documentation that says how far you can drive without damage. I'm sure that it's out there, however.

Basically, the less your differential has to work, the happier it is and the longer it will last. Making it extreme, with a small spare, will harm it more than running tires with %50 tread on one side versus %70 on the other, when we're talking millimeters versus centimeters.

Dave

[ciper]

If we had locked differentials, or open differentials it wouldnt be so bad. With open you it wouldnt even cause an issue but performance wouldnt be so great.

The problem is worse on the 4eat. With different size tires you are constantly slipping the clutch. At least in the 5mt you are only heating the fluid which is more slow to break down.

[rallysam]

I searched through archives on a lot of forums, and couldn't find much consensus.

Subaru says that the *circumference* of each tire should match within 1/4". Convert that to tread depth (radius) and that's 1mm. That is absurd in so many ways that I won't even bother ranting about it. But, no body seems to have better numbers aside from "I heard about this one guy who...."

Anyways, 300 miles is clearly too far for the donut, but sounds doable with a full-size spare. Please let me know if that's sketchy. Maybe I'll get my full-sized shaved down a bit so it's not super-fresh.

Sam
90 Legacy L Wagon AWD (5MT)

PS It sounds like I should give up on my other idea of rotating the full-size spare on and off with the other tires to keep all 5 evenly worn. Too much long-term stress to the diff. Instead, when the time comes, I'll just risk the short-term stress of one long trip home on a fresh spare.

[ciper]

Well, since you have open front and rear diffs you can put two large tires on the opposite ends and the problem will be less.