Tires and all-wheel drive

Have a subaruru outback 2001 LL Bean edit. One tire was damaged due to a pothole, the remaining tires (all matching) still have great tread remaining. Supposedly thes model of Yokohama tire is discontinued so I cannot easily get a like replacement tire. Regional tire chain here in RI says I should replace all four tires to avoid damaging my differential. I replaced th tire for now with another brand of equal sizes and speed ratings for now to make it drivable. Thus my question would be is the tire chain correct or am I okay with the new tire?

Carefully measure the circumference of your new tire and one of your other tires. That measurement should be within one inch of each other. Your AWD system should be able to tolerate that slight difference. More than that, and you a flirting with disaster.

We Have Recently Debated This Problem Again And Again And Again.

Here are a few links to click. There are more if you use “search”.

The answer lies in your Owner’s Manual.

Reach over to the glove compartment, take out the manual, and read what the manufacturer has to say on this issue. No matter how well-intentioned people on this site may be, their opinion is secondary to whatever the car’s manufacturer states regarding the issue of tire circumference.

Subarus are generally very sensitive to circumference differences and damage can be done. Your owner’s manual should have the details. In addition, you really don’t want different tires with presumably different traction characteristics on the same axle. I’d change to all new tires and try to sell the used ones to other Outback owners who are looking for replacement tires (which may need to be shaved).

Make sure tire pressures are correct on all four tires.

Measure the circumference around the new(odd) tire and note. Measure the circumference around the other three tires and average. These odd tire vs average of 3 existing must be within 1/4" otherwise you will damage your center differential possibly.

How many miles on the three tires that weren’t damaged?

As others have stated, Subarus are particularly sensitive to differences in tire circumference. It’s the price you pay for such a sophisticated AWD system, and there can be expensive damage to the drive train if the tires are not REALLY CLOSE in circumference.

Check your owner’s manual for more information.

with a discontinued tire, and a mismatched 4th(just because they’re the same size and speed rating doesn’t mean they’re good together. I’d just get 4 new tires and drive happy knowing you don’t have to worry about replacing for a little while.

What do you base that 1/4 inch on?? I suspect a set of brand new identical tires will vary more that 1/4 inch in circumference. These AWD systems have center differentials whose job it is to absorb slight differences in axle rotational speed, which is occurring all the time as you drive around curves and make turns. One revolution per mile isn’t going to damage them…You guys are getting carried away with this AWD/tire stuff. It’s turned into a HUGE tire sales tool…

One revolution per mile isn’t going to damage them.

A quick calculation seems to indicate that a quarter inch variation will give about 36 revolutions per mile difference.

36 revs/mile differences? I get 11 revs/mile (difference 947 vs. 935 rev/mile) using a 225/60R16 tire with a 0.25 increase in diameter. That’s a whopping 1.2% difference.

If Subaru’s diffs can’t handle that then we would be seeing a huge number of diff failures due to unequal tire inflation and people who take lots of corners:)

I agree with you for three reasons … but …

Reason 1. When I cocktail napkined the numbers, I worked out that a .25 inch difference in RADIUS was about equivalent to driving a road with four hairpin bends per mile. (.25 inches of radius ~= 1.5 inches of circumference) My rough calculations aren’t always correct of course.

Reason 2. There are a lot of Subarus in Vermont, and I doubt that more than half a dozen owners even know they have an owner’s manual, much less read the thing. Many folks would put a tractor tire on the car if the bolt holes matched and the car tire was flat. And some’d leave it there. It’s a safe bet there are a lot of Subarus in this state with substantially mismatched tires. The roadsides are not littered with dead Subarus.

Reason 3. If I’m not mistaken, Subarus come with the usual small diameter limited duty spare. Would they really ship the car with a spare tire that is virtually guaranteed to destroy the drivetrain if the driver does not disable the AWD or fails to disable it properly? I’m inclined to think not.

BUT … It isn’t my car that is going to need a couple of thousand bucks worth of work if we’re wrong about this. And it isn’t going to be my car either. AWD drive has some appeal. But four new tires if I hit the wrong object in the road doesn’t. No Subarus in this household.

While I agree with you in general, I’m not sure that underinflation changes the circumference of a tire. I think maybe it just makes the tire less round.

Tires do not vary by a quarter of an inch if same make/model/size. Maybe in the old days way back when.

The # is Subaru’s recommendation here-> look at #4.

It will not destroy the driveline but put excess wear on it leading to premature failure. Beyond that the manual transmission AWD is much more tolerant to tire differences than the automatic transmission AWD. Subaru just takes a single stance to eliminate more confusion.

AWD failure in manual transmission Subaru is pretty rare. With automatics where the tolerances really count as suggested by Subaru it matters. The AWD system is completely different for automatic(electronics/clutch pack) vs the manual(pure mechanical system) transmissions.

Most VT’s likely have the manual transmission Subaru as a guess and don’t run across the issues. My family has 10 Subaru’s all manual except one and never any AWD issues over 50k-250k miles.

On the spare, the automatic AWD has a disabling jumper(fuse) that you are supposed to install when a spare is placed on. The manuals do not as it works simply on fluid shear and is more tolerant. They only limit the travel distance to 50 miles on spare which is common sense across any vehicle on a donut.