mix and match tires with all wheel drive?

August 16, 2011 12:05 PM

I have a all wheel drive vehicle. A number of years ago all four tires were replaced , they have never been rotated. Now the fronts will not pass inspection but the rears probably have about 40% of tread left. I understand that one can ruin the transfer case if one runs different sized tires on this and many other all wheel drives. I would like to buy two of the exact same tires, which are still available, and put those on the back and put the old tires on the front. Do you think that the difference in diameter between the new tires and the partially worn ones will cause a problem with the drive train?
posted by flummox to travel & transportation (14 answers total)

You want the best tires on the front - the reason they wore out sooner is that they take the brunt of the wear when you turn -- so you want the new tires with the best tread on the front of the car to help you maintain your line in turns -- talk to your mechanic, but it shouldn't make a difference on the wear of the tires (new versus old) as long as the front and rear are matched up in pairs.
posted by cactus86 at 12:16 PM on August 16, 2011

You can have new tires shaved to match treadwear if necessary.
posted by mkb at 12:16 PM on August 16, 2011

I believe that the wear on the tires can make a difference for AWD vehicles (it isn't such a problem for 2wd or 4wd but AWD is more finicky). I have specifically heard that this is true for Subarus.
posted by fieldtrip at 12:17 PM on August 16, 2011 [1 favorite]

The car people I know all say "not a good idea." You get to make the call (hey, your car lasted this long with mis-matched circumference tires due to the wear, right), but think about this: for each millimeter difference in circumference between front and rear tires, the planet gear in the center diff will have to continually rotate an equivalent amount to tolerate that difference between how far the front and rear traveled. That gear, during normal perfect operation, doesn't rotate at all. It's typically sized and built to handle reasonable work (some slipping in snow, rain, typical tire wear, etc.).

So the question you have to ask yourself is this: how much are you willing to gamble that you'll never wear out that center diff with the extra work you're adding to its job? It's a very small amount of work, but remember that it is constant while driving, not just a little work during tire slippage. Maybe you put on mis-matched tires and, because your center diff was manufactured with good tolerances, nothing ever goes wrong. Maybe you got a unit with a perfectly workable but marginal bearing and the extra work trashes the thing.

How much does a diff rebuild cost versus 40% of two additional tires ('cause your only losing out on that much tire life compared to your current set at 40%)?

Also, hopefully you'll rotate the new tires, knowing now how much of an annoyance it causes when one pair wears out early.
posted by introp at 12:19 PM on August 16, 2011

If the tires are more than 5 years old or so, I suggest replacing them all. Tire age is a better indicator of a tire than tread wear. In fact, states are beginning to adopt safety laws that do not allow for the sale of tires more than 5 or 6 years old even those that are unused and have been in storage.

One of the reasons for replacing aged tires is the loss of the competitive absorber (blended into the rubber) due to sunlight exposure. The absorbers compete with the rubber to out-absorb UV and in-turn protect the rubber from UV. This is somewhat like a sunscreen does on your skin. Once the competitive absorber is depleted, the rubber ages—rots—fairly quickly.

Aged tires fail and, let alone transfer case issues, that is enough for me to replace tires on a calendar basis.
posted by bz at 12:53 PM on August 16, 2011

Most places offer a 4-for-the-price-of-3 deal, so you're probably only saving the price of one tire. And if you do what you propose, you'll be right back in the same situation the next time you need two new tires. Buy the 4 new wheels and rotate them every other oil change.
posted by yerfatma at 1:12 PM on August 16, 2011

cactus86 writes "You want the best tires on the front"

No. Your best tires should go to the rear for any kind of normal driving (IE:99.999%) regardless of which wheels are the drive wheels.
posted by Mitheral at 1:22 PM on August 16, 2011 [1 favorite]

I've had 2 AWD drive cars and the rule has always been that if you replace one tire, you replace them all. Full stop. I'm going to assume that most people who are giving you advice about where to put your good tires either missed the part about your car being AWD or are ignorant to this rule for AWD vehicles.

Once, I got an unfixable flat on one tire and had to buy a complete set. Sucks, but that's the trade-off you get along with the benefits of AWD.

Like introp says, it's a big risk to do what you're proposing. I'd be surprised if you could find somewhere that would put these tires on for you without, at the very least, strongly cautioning against that.
posted by moojoose at 1:57 PM on August 16, 2011

You have been running mismatched tires for quite some time now, as your fronts are worn enough to fail inspection and your rears are around 40%. So putting the 40%ers on the front and new ones on the rear will increase the differential between the diameters, but you've already got a differential.

Having said that: the safest and most cost-effective solution is four tires, because buying two will just guarantee you wear out the 40%ers when the new ones are fairly warn, then you'll be in the same boat again. Suck it up, buy four tires, rotate them regularly, and keep the two worn ones around for spares (like if you have a blowout when you have 40% left on the four new tires.)
posted by davejay at 3:48 PM on August 16, 2011

Oh, and if it helps you sleep at night: think of the cost of the extra two tires as roughly equivalent to the money you saved by not rotating the tires, so you didn't save or lose any money. :D
posted by davejay at 3:50 PM on August 16, 2011

You want the best tires on the front

Just reiterating that this is false, in case anyone missed that. You want the best traction on the rear, because understeer is preferable to oversteer in rain and snow.
posted by davejay at 3:51 PM on August 16, 2011

I neglected to mention that when I purchased the tires I bought 5 tires, the extra one for the spare, and had planed to rotate the spare into the mix. Unfortunately I never bought the expensive 5th alloy rim so never rotated that "spare" tire into the mix and thus have one brand new tire already. If I had to do it over again I would not buy these tires either....
posted by flummox at 3:51 PM on August 16, 2011

I have an AWD Ford Edge. I just got my first set of replacement tires at 45K miles and I only rotated them once for all those miles. It is true that my front tires were the most worn. I agree with yetfatma that you should bite the bullet and buy four new tires to synchronize them all. The rotate every 10-20K miles. The cost of new tires is negligible compared to drive-line repairs, and you'll sleep better and have better sex too.
posted by nogero at 5:59 PM on August 16, 2011

Replace all four tires, get them rotated periodically (or buy a jack, jack-stands, and a torque wrench and learn to do it yourself), and leave the spare as-is.
posted by 6550 at 1:43 AM on August 17, 2011