Replacing tires on an AWD vehicle

We have a 2001 Subaru Outback named Ruby Sue. About 6 months ago, we got a flat tire and took it into Discount Tire to have it repaired. The tire expert there said that we should replace it for our safety. Not wanting to put our lives in danger we agreed that the tread did look a little thin and asked him to replace the tire. He then told us that because we have an all-wheel-drive vehicle we would need to replace all of the tires. We figured that if one was worn down to the point of needing to be replaced, probably all four were too.

Fast forward 6 months, we get another flat tire which needs to be replaced due to damage to the side wall. I figured that because the tires were relatively new, we could substitute a brand new one without any problems. The nice salesman at the tire shop wanted to sell me the replacement warranty for all four tires. It’s a good idea if you don’t have to replace all four but a sucker’s deal if you do as it will only cover one tire if it blows.

So the question is, “In an AWD vehicle, do all four tires need to be at the same? Can one replace just one tire or even just one pair at a time?”

So I did a little searching on the forum and found another with the same question. Sounds like there would be an issue with the center differential if the difference in tread is too great from one tire to the next which would affect performance and end up costing more in the end. But would it be ok to have the front tires at one tread level and the rear tires at another? If not, and all four have to be always at the same level, what is the acceptable difference in tread level that can exist?
If we keep getting flat tires that have to be replaced, we might go broke replacing all four.

I would think that your owners manual would have something to say on this. Have you looked?

If needed the folks at can shave a new tire down to match the existing tires.

I’ve been quite happy with the service & prices that i’ve received from the folks at

Buy the same brand and model tire and you will be fine. If you do this, the only discrepancy will be the difference in wear between the tires. If we assume you have at least 50% tread on your old tires, or about 5/32, then the resulting difference on the radius is only 5/32in, which is only 1.2%. This is well within what the differentials can cope with. I assumed a 205/60-15 tire, which is common for Subaru?s. If you have the larger tires the percent difference is even less.

In your case given short driving period I believe buying the exact model/make/size(locate if discountinued) will do.

Subaru’s rule is here->

I agree. Six months of driving should not have caused an excessive amount of wear on the remaining 3 tires, so the difference between the new one and the “old” ones should be minimal enough so that you won’t have any problems.

According to the Subaru web site (, the rolling circumference of all four tires must be the same within 1/4 inch. This is to prevent damage to the all wheel drive (probably the differentials).

A 205/60-15 tire has a circumference of about 78 inches. Therefore, the tires must be the same within 0.25/78, which is about one third of one percent. To put it another way, the acceptable difference in tread depth is only one twelfth of an inch.

We also own a Subaru, a 1998 Legacy GT wagon. We are on our fourth set of tires. The first set wore out. The second and third sets had to be replaced when half worn because one tire had an unrepairable flat. We recently had a flat with the fourth set, but caught it before the tire was ruined.

Correction! The acceptable difference in tread depth is only 0.040 inch, that is, one twenty-fifth of an inch. (I forgot to use radius instead of diameter.)

Unless you only travel in a straight line, the front wheels will always travel further than the rear ones. The center differential is designed to take care of that. For that reason, it would be OK to only have both the rear tires the same, and both the front tires the same diameter (degree of wear). In other words, you don’t need 4 new tires all at once. I would have the front tires with the most tread, since they do most of the braking. Consult your manual anyway, but don’t listen to tire salesmen. Since you have so many flats, it would be folly to keep buying 4 tires at the same time. When you buy 2 new tires, keep the good one of the pair you replaced. At some stage you will enough extra tires to be able to mix and match to get 2 equal ones on the front and two equal ones on the back. I do recommend you stick to the same basic tread pattern and rubber compound. Do not mix ice radials with all season radials, for instance, since their traction and braking performance differs greatly.

Does all of this hold true for non AWD vehicles? Can you replace just one tire on a regular front wheel drive car and not have a problem. A difference in tread of 0.040 seems really really small. I love my little Ruby Sue, but tires aren’t cheap.

No problem with front wheel drive or rear wheel drive.

I think it might help if you told us how many miles you have on your tires, and whether it was a front or back tire that had the flat.

Agree; the difference in diameters is so small that the differential does not need to work very hard. If you live in a city, the differential is working all day long making all these turns. A city taxi can go 1,000,000 miles without replacing its differential.

It was the front driver side tire that was replaced.
What I am really worried about though is when we are halfway through the life of the tires and a flat happens. That means I don’t get all of the value of the tire that I paid for if we have to replace all four. If we can get away with replacing just 2, well that’s a little better but we’re still wasting one half good one.
DocNick was the only one to address this issue of having 2 different pairs front and back. Does it sound ok to the rest of you guys that we could have that situation?

Well hopefully you will have repairable flats. In my experience, holes in the tread are much more frequent than sidewall punctures or tread separation, if you’re just driving paved or mild gravel roads. In the 4 sets of tires I’ve had on my Thunderbird over the years, I have had 4 flat tires due to punctures and they were all repaired reliably.

.040" is not enough tread wear to worry about. The differentials can handle that without any problems. That’s why they install them, to allow the tires to rotate at slightly different speeds.

AWD is an expensive feature most people can live without. And it just keeps getting MORE expensive as time goes on.

I have no idea how much of a difference the differentials can handle. I just know how to read Subaru’s position in the link I included. The limit they give is 1/4 inch of rolling circumference. The corresponding difference in radius is 0.25 / (2 x Pi) = 0.040 inches. This is a bit more than 1/32 inch. If Subaru had not backed up my tire dealer’s advice that mixing new and worn tires will damage the all wheel drive, I would not have scrapped three serviceable tires on two separate occasions. Other makes with all wheel drive may be more tolerant.

As far as living without all wheel drive is concerned, it depends on what you have to drive in. After a winter ice or snow storm, there are only two things that stop my Subaru. The minor one is snow deep enough to high center the car; then, I just stay home. The major one is all of the non-AWD cars, on all season tires, that get stuck in front of me.

It is not ok to have tire pairs of different size on the front and back of your Subaru.

Unless your front tire’s tread depth is within .04 of new, then you should have your new tire shaved to match the existing one. As mentioned, Tire Rack is one place that will shave tires to your spec.

If all your tires do not match, you will accelerate the wear on your drive system.

I think caddyman stated most can live without AWD which I agree. I personally cannot due a family estate(we use as ski house) with poor access/priority on plowing pecking order.

One thing people do not realize is company’s like sell bulk of tires at discount prices and will shave(wear down) a single tire to match tread depth for a fee ($20 I believe). Far cheaper than replacing four tires. Local company’s may shave tires too but is fast and quick and relatively inexpensive.

I would have the front tires with the most tread, since they do most of the braking.

No, the best tires should be on the rear, to minimize the chances of the back end breaking loose and swapping ends. Seems strange, but that’s what the experts are saying these days.