Replacing a Tire on an AWD Car

October 26, 2023 5:28 PM

Can a kind automotive MeFite please straighten me out on this “rule” the internet told me about tires and AWD vehicles.

According to the internet, if you have to replace one tire on an AWD car (like, say, one gets an unrepairable puncture) you will need to replace all four at the same time.

(Note: I’m not talking about using spare tires. This is only about purchasing a replacement tire for a damaged tire.)

The supposed logic there being that the new tire (even one of the same make/model) will, due to it not being as worn as the remaining three tires, throw-off the balance of the AWD system and result in all hell breaking loose. Assumedly, destroying drive shafts/bearings/whatnot.

I find this really hard to believe, but some folks seem to hold this as a hard truth.

Is there any truth to this? I can understand doing so if the tires are super worn, but not with moderate mileage.

Thanks.

posted by Thorzdad to Travel & Transportation (9 answers total)

I've been told this as well; the tires have to be nearly the same diameter for the AWD system to work. Here's something from the Subaru Owner's forum: https://www.subaruoutback.org/threads/nail-punctured-1-tire-do-i-need-to-replace-all-4.349241/ Note the comment about the tolerance needed being listed in the owner's manual.

Most big tire shops can shave the new tire down to match the others, if they're worn enough that this is needed.
posted by ReferenceDesk at 5:35 PM on October 26, 2023 [2 favorites]

I replaced a damaged tire on an AWD vehicle a few months back and the tire shop based their recommendation off of available tread in the remaining tires. I only purchased one new tire.
posted by Pantengliopoli at 5:36 PM on October 26, 2023

Consumer Reports says this is only SOMETIMES necessary, and you can have the new tire shaved down to match the other "used" tires.
posted by kschang at 6:41 PM on October 26, 2023 [2 favorites]

Consult the manual for the vehicle in question, which should specify the allowable difference in tire diameter. This can be different for different vehicles, as not all AWD systems are the same.
posted by ssg at 7:04 PM on October 26, 2023 [1 favorite]

Definitely not a requirement on Tesla AWD EVs (the front and rear axles are driven by independent motors and are not mechanically linked). Tesla will replace just the front or rear pair unless all 4 are worn.
posted by zippy at 7:28 PM on October 26, 2023

I’ve had to replace the center differential on a Subaru forester at 75k miles, presumably because we previously override the recommended change of all 4 tires when 1 failed. The center diff cost about twice what a set of four tires does.
posted by u2604ab at 8:46 PM on October 26, 2023 [2 favorites]

he supposed logic there being that the new tire (even one of the same make/model) will, due to it not being as worn as the remaining three tires, throw-off the balance of the AWD system and result in all hell breaking loose. Assumedly, destroying drive shafts/bearings/whatnot.

I find this really hard to believe, but some folks seem to hold this as a hard truth.

Two-wheel-drive vehicles include a differential ("diff") in the drive train, whose job is to allow the driving wheel on the left side of the car to rotate at a different rate from the one on the right side so that the car can go around bends without wheel slip while still applying driving force to both wheels.

Most of the time the car will be driving something very close to a straight course, which means that the parts of the diff that actually allow the wheels to rotate at differing rates have nothing to do and suffer no wear. Diffs are built with that in mind: they're simply not engineered to cope with supporting unequal wheel rotation rates for the whole time the car is in motion.

If you put tyres with different diameters on the left and right driving wheels, then as well as rotating at different rates when driving around bends, those wheels will rotate at different rates when driving a straight course. Which means that the diff has to do its thing continuously, and will wear way way faster than it's expected to.

Four-wheel-drive vehicles have three diffs - one for the front wheels, one for the back wheels, and a third between the front and back pairs. Again, this is done so that minor and temporary differences in wheel speeds caused by terrain irregularities can be accommodated without forcing tyres to slip or putting excessive stress on drive shafts.

Putting just one oversized wheel on a four-wheel-drive car will make two out of the three diffs compensate continuously for that wheel's lower rotation rate, causing more wear than usual. Putting on a pair of oversized wheels either in front or in back will promote wear in just the central diff, because the front and rear pairs will be rolling at different RPMs.

Any hell that breaks loose does so when the diffs wear out, not as an immediate consequence of the oversized wheel. The point is that forcing parts that are simply not designed to work continuously to do just that puts wear on them at a much much higher than normal rate. And, as noted above, replacing worn-out diffs takes longer and costs more than replacing tyres.

This is also why regularly rotating the tyres in accordance with the car maker's recommended pattern, so that the tyres wear down at roughly equal rates, is even more important on a 4WD than on a 2WD.
posted by flabdablet at 12:09 AM on October 27, 2023 [22 favorites]

Great answer, flabdablet. As others have said, a good tire shop should be able to shave down one new tire to match the diameter of the other three, but you still may get a strong recommendation to replace all four, especially if the other three tires are substantially worn. It may sound like they're trying to upsell you on a full set of tires, but it may be more of a case of "Why remove and waste 80% of this brand new tire's life, when your other three are due for replacement soon anyway?"

There should be information about allowable tolerances in your vehicle owner's manual; if not or if you cannot find the information, check with your local library. With many public libraries, your library card lets you not only check out books, but access a huge amount of online resources, including online auto repair manuals and information.
posted by xedrik at 9:09 AM on October 27, 2023

A really good tyre shop will let you trade in your partially worn but otherwise undamaged tyres for a discount on any new set, and offer second-hand tyres with a known degree of tread wear as a lower-cost single-wheel replacement option for exactly this use case.
posted by flabdablet at 10:01 AM on October 27, 2023 [1 favorite]