I had all 4 tires replaced 18,548 miles ago, which was 1 year and 1 week. 1 went totally flat yesterday with a big chunk of metal in the middle of the tread – the visible part of the metal is about the size of a razor blade but much thicker, and I don’t know what the shape looks like down inside the rubber. If I’m lucky the part that penetrated is quite small, but for this post let’s say it’s big and the tire is unrepairable. What then?
Do I replace 1 or 2 or 4 tires?
The car is a Subaru Forester with “All-Wheel Drive”. When I replaced the tires a year ago, they weren’t bald, and it wasn’t visually obvious to me they needed replacement, but the tread depth was “below minimum specifications”. I remember discussing that it was a good time to replace them because winter was coming. They had 66,411 miles on them. If that represents 100% of tire life, on a milage basis my current tires are 27.9% worn. I measured two and found the current tread depth 0.298” and 0.269”.
By the way, when the tires were 5 months and 10100 miles old, one caught a nail and was unrepairable, and the Subaru dealer said it was fine to replace just that one.
Looking around online I find some range of advice that could justify 1 or 2 or 4 replacements. I will consider what the mechanics say when they’re open, but for my planning purposes: What say the Dopers?
Yeah, probably. The tire shop can take a measurement of tread wear and give a recommendation but it will almost certainly be that there is too much mismatch which will put stress on the AWD system. They can “shave” the tire down, which will be pretty expensive for that much removal but still far cheaper than buying a full set of tires. TireRack will actually shave tires to and depth for a flat fee ($25 or $35 dollars, I forget) so you could order from there and have it drop-shipped to an installer.
I did something like that with an Outback, a single tire replacement, however I was up over 250,000 miles. It didn’t cause any problems, but again I think I had less miles on it then you did, perhaps 10k, or slightly less. Over time I kept that tire on the front, while rotating the others to the back which caused a little bit of more wear to help it even out faster.
IIRC there is a tire depth measurement that would be able to tell if the mismatch is tolerable.
The instructions for using a donut for my Subaru would be the donut goes on the back, and the car put into FWD mode by installing a fuse in a certain slot. So a flat in the front means removing the rear tire, replacing the donut, then removing the front flat tire and putting on the former rear.
I read this. I think I read that if the wear is more than 4/32" (? an odd way of saying 1/8" ?) it’s too big a mismatch. I don’t know if I’m there because I don’t know what the new tread depth is.
Hate to say it, but in this particular kind of situation in lieu of overly complicated tire-rotation schemes and other questionable maintenance practices, the only sensible option really is to just buy a new car. Tough break. Is the ash-tray full, perchance?
Can someone explain to me what it is about AWD that is so sensitive to differences in fractions of an inch tread depth? What kind of damage are you looking at if things don’t match up?
About 20 years ago I had a Mitsubishi Eclipse with AWD. I had it for about three years and in that time I had three flats. I always just replaced the flat tire, and nobody at the time ever said anything about the tread depth having to match the other tires. Were things different back then?
Not really, but I’m not familiar with the Mitsubishi systems. On Subarus, the recommendation has been there for a long time. AWD systems are designed to have all four wheels spin the same, they are balanced a few different ways. If they are off, even by a little, the transfer case and gears will be constantly adjusting, causing strain on the systems.
Granted, most of the advice to buy 4 new tires is given by people who have a vested interest in selling you tires. I’m sure there’s a threashold below which it’s not noticeable, but it appears to be a real issue for anything but fairly new tires.
Everything with brakes - and similar, work is ideally performed as axle pairs. I don’t know how the AWD system is monitored, but surely specific RPM is part of it. Could it be the system is precise enough where 1/4” tire diameter difference between 2 (or 4) tires is enough to confuzzle the vehicle traction computer? Wouldn’t surprise me.
The limited slip center differential or clutch pack which distributes power between front and rear will have to work continuously to compensate for an imbalance (i.e. it will ‘perceive’ the vehicle to be constantly turning) which can prematurely wear gearing or burn out the clutch. The same is somewhat true for front or rear limited slip differentials even on 2WD vehicles. It can also cause handling problems, although unless you are doing high performance driving you’ll probably never feel the difference.
How much of a problem this is depends on the degree of mismatch and how much highway driving you do (and the diameter of the tire) but it is generally a good idea to keep the tires even in wear by regular rotation and shaving down a replacement tire to match the others if there is too much difference.
Since you had the option of buying one new tire and shaving it down and since your tires are lasting 66,000 miles while these have only 18,000 miles it sounds like you threw about $400 or so into the garbage.
I didn’t find any easily available option to shave one tire down. The places I do business with and trust didn’t know of a vendor for this. I have a busy schedule and live half an hour from grocery, pharmacy, doctors, veterinarian, and of course the repair shops and tire shops, so it is a bit of a problem to have my car compromised. I spent four hours dealing with this, WITHOUT finding and vetting a new to me vendor to shave down a tire. So I spent $300 in my time plus the cash outlay. It doesn’t look to me like a sure thing that I could have saved overall by pushing further on the shaving possibility.
And besides, what I find online about tire shaving doesn’t sound so great:
I’m highly skeptical of slight differences in tire circumference due to replacing 1 new tire or a mismatched tire negatively affecting the drivetrain. Of course tire manufacturers push this claim because it means more tire sales for them. Consider that 90% of all tires on the road are not at their correct pressure or matching pressure with the other tires at any given moment. A tire not at correct pressure has much more influence on its circumference than tread depth differences of fractions of an inch.