I know that with an all-wheel drive vehicle, you have to keep the tire tread depth about the same. and, if you need to replace a tire when the others are partially worn, you should replace all 4.
But isn’t there an alternative? It should be possible to remove enough tread from the new tire so that it matches the other 3. Can tire dealers do this? Certainly a cheaper alternative, specially if the others are only, say, 1/4 worn.
If not, this is a great idea for a machine to perform this, just shave a few 16ths from tires…
Tires can be “shaved”, but it’s not a very common practice these days outside of competition tires. Most tire shops don’t do it. And any warranty the tire may have goes out the window when you shave the tire.
The real problem here is the tolerance a given AWD system has to difference in tire diameter. Some systems are very unforgiving. Some are very forgiving.
And what comes along with the unforgiving nature of some AWD systems is that rarely are owners informed BEFORE they purchase the vehicle. It’s only afterwards that they become aware of the situation and have to deal with the possibility of purchasing 4 tires instead of just one.
The real problem here is the tolerance a given AWD system has to difference in tire diameter. Some systems are very unforgiving. Some are very forgiving.
And what comes along with the unforgiving nature of some AWD systems is that rarely are owners informed BEFORE they purchase the vehicle. It’s only afterwards that they become aware of the situation and have to deal with the possibility of purchasing 4 tires instead of just one.
True, some AWD systems seems to work fine with 4 mismatched tires and others will fall apart with one oddball tire in a short time. But for me, I recommend no more than 3/32" difference in tread depth of the same brand tire on an AWD system. And there are options other than buying 4 new tires. The used tire trade has taken off like wildfire and there is probably a used tire that will suit your needs available. There are still tire shops that shave tires to match too.
As for not being aware of needing to maintain equal tread depth, well, buyer beware.
“As for not being aware of needing to maintain equal tread depth, well, buyer beware.”
IMHO, it’s not much different from buying a car with a timing belt and ignoring (or not being aware of) the need to change it by the specified mileage/time interval. Unfortunately, there are always going to be people who select vehicles w/o having any technical knowledge–or any interest in gaining any of that knowledge–and their wallets suffer as a result.
All it takes in order to learn about issues like this is to read the Owner’s Manual, but as the veterans of this board are only too aware, that little book is a widely-published, freely-provided publication that gets few readers.
What’s that old saying?
Isn’t it something along the lines of Knowledge is power?
JTSanders said: "@CapriRacer, rank some AWD systems with respect to tolerance for different diameter tires. "
Sorry, but my expertise is in the engineering and usage of tires. I am only aware of AWD system differences simply because of web sites such as this one. As an engineer, I can understand how the different kinds work, and why some would be more unforgiving to tire diameter, I don’t have the background to say which ones are forgiving. I can only tell you of the ones I’ve heard that are problems: Subaru and Volvo. There are likely more, but those are the ones that leap into my head.
So the rule of thumb to prevent issues with AWD is to match all the tires as best you can. Usually this mean the same make and model, and the same state of wear, rotating regularly so the wear rates even themselves out. This will prevent any issues from surfacing.
But this also means that sometimes this is unnecessary. You won’t know until you get a failure, and that would be too late!
An added thought to all the good ones so far. AWD systems are quite tolerant to up to 2/32 inch. With out some tolerance built in, they would self distruct when loaded in the back only or on one side or when driven over uneven terrain. Slight variations in tire size increases the wear on the differential gears less then bigger differences. It isn’t like some variation will kill a car in just a short time. It’s always a combination of factors. If you are in the habit for example, of never monitoring and doing something about unequal air pressure, that can wear your AWD differential much more then a slight difference in tire size. If you trade your vehicles in before 100k and have no remorse for the next guy, you can get away with more still.
In fact, the center differential of an AWD deserves the same respect on tires compared front to back as everyone has been taught to give to the front or rear differential on tires from side to side in 2 wd cars; no more, no less. When you own an AWD car, just keep that in mind and you will be fine. I think everyone agrees with general guidelines for equipping 2wd cars with tires in pairs on the same axle. The same is true front to back for AWD cars. It’s just a basic concept that when adhered to, maximizes the life of ANY differential. Keep in mind that any differential under harder use, which includes uneven or heavily laden tires or unequal sizes should be serviced more frequently.
I can’t speak for @VDCdriver or any other long time AWD or 4 wd owner but differential failures for these are no more frequent then the any 2wd car if they are sanely driven and maintained according to the manual. They are not to be feared but enjoyed for their optioned feature and maintained a little differently. If done so, they give a car life time length of service, just like any other mechanical component.
^
No, dagosa, I have never experienced any problems with differentials or center viscous couplers on any of my Subarus, but as we know from this forum, those problems do occur for those who choose to ignore the maintenance schedule for their car, or who never read the proviso in the Owner’s Manual regarding “matched” tires.
OK, thanks for your response, @CapriRacer. I appreciate an honest answer. I also appreciate the value in knowing your limits as an engineer, too. BTW, it wasn’t a trick question, just curiosity piqued by your post.
I don’t think the problem with AWD systems is the center differential. I think it is the viscous coupling or whatever device is there to control the amount of torque that is split front to rear - sort of like the clutch plates in a limited slip differential.
An open diff doesn’t provide torque to both sides, and an open center diff would work the same way. The problem with an open center diff would be that no torque split would be provided when the tires start to slip - which defeats the purpose of the AWD.
In classic 4X4 units with a transfer case, the front driveshaft is turning at the same diameter as the rear driveshaft - and so long as all of the components are stronger than the grip of the tires, there should only be tire slippage - even with severe differences in tire diameter. I don’t think I have ever heard of a classic 4X4 unit having an issue with tire diameter. The only time I have heard of issues has been with AWD units that have a viscous coupling - and it was the viscous coupling that overheated and failed.
Because of the difference in operation of an AWD with center differential vs a “classic” 4by4 with a transfer case and no center differential, I agree with @CapriRacer and you really can’t compare the wear tendencies of one to the other when different size tires are used. For one thing, truck based systems with transfer cases and no differential are just not operated the same way and wear caused by unequal tread size is not that much of a problem. Traditional truck based systems are only operated on slippery surfaces which mitigate the concern for differences in tread wear to begin with and operating them on dry pavement where they are locked, has a much greater effect then any difference in tread wear. The binding on these units when it occurs can cause failure or increased wear much more rapidly the any uneven tread wear and generally, owners don’t drive them that way.
That’s why @CapriRacer is right, and I would be hard pressed to worry one bit about making sure the treads marched on my truck based systems…little if any concern relative to any 4wd component problem, front to back. Maybe side to side though because of the rear differential, for sure !
The biggest reason for concern is for safety and unequal traction when driving older traditional transfer case on slippery roads in 4wd and the unequal traction and locked or no center differential can throw them into a spin in a heart beat on corners when going too fast. If I need locked 4 wd, I don’t travel over 35 to 40 mph…and that may be too fast. That is a big reason why some of these puppies are found in ditches to begin with in slippery weather…
Subarus seem particularly sensitive to mismatched tire diameters. We’ve seen several posts here from Subaru owners with ruined differentials due to that problem.
Honda CRV’s have a different AWD system and don’t seem to be prone to that issue, though. I have a '99 CRV and the owners manual does not even mention any need to match tire diameters.
Jesmed1 said: "…I have a '99 CRV and the owners manual does not even mention any need to match tire diameters. "
Be very careful here. Many owners manuals do not mention matching tires at all! 0 in spite of the fact that they have TSB’s and other warnings on the subject. I’ve never understood why.
@CapriRacer, my '99 CRV manual says specifically that the tires “may” be replaced in pairs (front pair or rear pair), with no reference to diameter matching. I seriously doubt they would have made that specific allowance if it could damage the differential. The way the CRV differential is designed, there has to be significant difference between front and rear wheel speeds for the rear differential to lock up, and minor variation in tire size is not enough to cause lockup.
Having said that, I agree with your point that manuals often omit critical information! But I have looked around and never seen an instance of CRV differential damage due to mismatched tires. Usually it’s failure to change the differential fluid that causes the problem on CRV’s.