@NYBo
With all due respect, I have to disagree.
There are three differentials in an Awd vehicle, and at least two are involved when one tire differs in diameter from the rest. The center, and the differential on the same axle that has to work harder dealing with that difference.
@Caddyman
I agree. But there are many of us who accept that added cost and maintenance because of where we live, activities we like and the jobs we do. Does putting on one tire with a different diameter differ in the require ment for an Awd car then it is for a fwd or rwd car ? Heck no. Over time, the results are more expensive in awd and more people seem to get away with it with 2wd. But if you do what you are suppose to with Awd, which is MINIMAL, and buy a vehicle from a reliable manufacturer, you will have no more added expense for drive train repair then most 2wd cars which are treated the same way. Now, if you drive Awd cars where they weren’t ment to go, all bets are off.
That’s one of the reasons I’m still debating AWD. If I’m 600 miles from home with 10K on the tires and I ruin a tire on a Sunday, what are my options? Buy four new tires at a Walmart? Tow to the nearest dealer and stay overnight? I’m not sure but it is a concern. With a FWD, I can buy a cheap or used tire to get me home and just make sure the mis-match is on the back. I’ve had to do it before 200 miles from home, so its not somethig outside of the realm of possibilities.
As far as Discount Tires go, I don’t disagree that they should have strongly advised against the one tire on an AWD, but I had one experience with the shop in St. Paul that sourerd me on them as a chain. I don’t remember exactly what the issue was but they either had some kind of a special balancing promotion or free rotation or something, and I either had a noise or a bump that I wanted to isolate to either tires or something else. Still had some life left on the tires but knew I’d need some new ones at some point. At any rate after two trips to them and waiting for over an hour, finally the guy starts asking if my tires were ones or twos or threes? Huh? He said if they are threes the guys won’t even balance or rotate them-they wouldn’t do it. I said how would I know and what difference does it make, its my car? So waited another 20 minutes and just left. Certainly whether right or not, they seem to be lacking in general customer relations skills.
I would agree with the store if they have no way to shave down the new tire to the right diameter. I point this out to all Subaru owners, especially those who are off the beaten track often.
@dagosa : The front and rear diffs are designed deal with different rotational speeds of the attached tires all the time; it happens every time you go around a corner.The center diff, or more typically, viscous coupling, has to deal with much less of a difference unless there is a tire size mismatch. Not a terrible problem for a normal diff, but it cooks a viscous coupling. Now, the way viscous couplings are controlled varies from manufacturer to manufacturer. Subarus, which use a few different systems, are extremely sensitive to tire size, with specific warnings about degree of wear difference allowable. My Chrysler, on the other hand, is a little less so; the manual only states you must use the same size tires with no specification of allowable wear difference.
I just read the g35x 2008 Infiniti owners manual and the wording on page 378 states:
If excessive tire wear is found, it is recommended that all four tires
be replaced with tires of the same size, brand, construction and tread pattern.
In fairness to the OP who asked the question, when the owners manual is close to 500 pages, it’s often hard to notice every detail.
When we bought our last car and I made a comment about the manual as the salesman handed it to me, he replied with “You’re kidding me. You’re going to actually read this?”
I can’t speak for the Infiniti manual on this car but in regards to my lengthy Lincoln owners manual, about 2/3 of it is warning disclaimers for everything under the sun.
“WARNING! Do not place head on upper radiator support and have someone slam the hood closed.”.
That’s being a bit facetious but there’s a lawsuit to cover anything the imagination can conjur up.
There’s also a bit of an OOPS moment when the Lincoln Mark VIIIs first appeared; the owners manuals stated that the transmissions should be serviced with Dexron II…
“Recommend” is not “Must”. You should have been ok with one tire. Apparently your Infiniti can tolerate one new tire. A new tire will wear faster than a used one to approach matching the diameters of the old tires. The weasel wording is likely there for legal reasons in case of an accident where a trial lawyer will grab whatever he can find to make his case. A different tire might have a different traction rating or even with equal traction ratings might have different traction as new compared to used. With antilock brakes, if you have them, this is moot.
I have a large vernier caliper that measures up to over three feet that I bought recently for about $10 at a discount store. If you did ultimately decided to change just one tire, allow me to suggest that you get one and compare the diameters of the new tire and the other-side worn tire. Diameters can vary amongst different manufacturers and even different tire models, and it would be a good idea to see if the diameter variation exceeds the manufacturer’s recommendation.
Someone in an earlier post suggested the tire-trimming machines that round tires out and can match diameters. I haven’t seen one anywhere since bias ply tires disappeared, but if you can find a shop that has one that might be an option if the diameters vary too much.
@TSM, I haven’t seen a shaving machine either, but tirerack.com will shave a tire to your spec if you buy one from them and pay extra for the shaving.
Just for curiosity, I checked the owners manual for my '99 CRV AWD. There is no recommendation / requirement that the tires be matching diameter. I love that thing. Easy to work on, reliable, and undemanding. But it is a dog, for sure.
@tsm “I have a large vernier caliper that measures up to over three feet that I bought recently”
What on Earth for? I’m a tool fan as much as the next guy but what could you possibly need to measure that’s 3 feet that you couldn’t use a tape measure or your thumb?
I’ve spent time trying to write disclaimers, warnings, cautionary notes, etc. - and it is difficult to get the wording right. The fact that the owners manual says “recommend” might mean “We don’t take responsibility if something breaks if you don’t follow our recommendation” - and the tire shop would be avoiding a potential problem by refusing to do anything that violates a recommendation.
And just so everyone understands, I have had several conversations with attorneys and a tire shop can’t avoid legal liability by having the customer sign a waiver. If I understand this correctly, the principle is about expertise and the responsibility for doing the right thing falls on the person who has the expertise.
@NYBo
I think you are under the mistaken assumption that all center differentials use viscous coupling when they do not. I would say the variation is many and viscous coupling is just one. Maybe a frequently used one, but hardly the vast majority in the most popular fwd cars made today. Regardless, being viscous coupled does not insure failure. Much like a torque converter, it theoretically is subject to less wear then gears if correctly made and calibrated. Some use Torsen limited slip, which ALL my Toyota Awds did which is direct variation of the standard axle differential. Honda for years never had a center differential and their so called “real time” was a clutch pack in the rear that responded to slippage on the front wheels with the clutch engaging in the rear differential.
No center differential per se what so ever. And as @jasmed noted, hardly influenced in normal operation by different tire diameters.
Read the article by Tire rack. Differences of up to 4/32 inch are acceptable by some manufacturers. With the average all season new with about 10/32 inch depth and 2/32 being the illegal limit, you realistically have 8/32 inch of tread wear to use. 4/32 is one half of your usable tread wear difference. That is pretty lenient and shows that manufacturers built cente differentials that are pretty robust. Look at consumer reports drive train problems for Awd compact SUVs. These are some of the most reliable vehicles made. I and my neighbors and all my friends who own Awd vehicles have never had a driveline failure. That is similar to the the frequency found in CR.
It is tantamount to scare tactics to keep saying that Awd cars from reliable vehicles ( especially Subarus) need anymore attention then you should provide to anycar, especially fwd cars which have minuscule front differentials and could not take running smaller diameter tires and even tires with different air pressure and loads at high speed without experiencing excessive wear. And, this is wear that may lead to a failure well after the original owner traded it in. I had two Subarus and both were very reliable…and I gave them no more attention then any other car other then maintenance of the two added differentials.
Everything wears a differential other then parking it. You use the same tire maintenance suggested in all manuals as you would for all cars, you have little to worry about. Using a smaller diameter tire on one side stresses both the center and the axle differential. It rotates the secondary gears responsible for the speed difference to prevent binding of the ring gear or primary drive gear in a center differential. If the primary drive is viscous coupled, it generates more heat when asked to account for the difference. Maintenance is the key to any differential long life as well as taking the same precautions concerning tire pressure, tread design and tire wear. It matters not which differential you talk about. None spontaneously self distruct as they all increase their wear til they fail earlier if you don’t follow manufacturers recommendations. That’s hardly any different then ANY CAR, 4 wd or not.
I agree with @CapriRacer. The fact is that an AWD car MAY be damaged by driving with one different circumference tire. The fact is that an oral or written disclaimer does not absolve a shop of any responsibility for resultant damage even if both parties think it does. There are numerous incidents of the generic story “Guy brings junkyard part to shop for installation. Said part fails, causes further damage, shop is found negligent for installing junkyard part because as the professional they should have known better.”
If an AWD car comes to me with a failed tire, I will install only one new tire only if the tread difference is less than 3/32". If the tread difference would be more than 3/32" it gets 4 tires, otherwise I will only install the spare. The reasoning is that they probably won’t drive too long on the spare. If I put one new tire, they might drive it until they come back with a noisy transfer case. I don’t want that problem.
We as shops are caught between trying to serve our customer’s needs as best as we can and trying to protect ourselves financially from the idiots out there who refuse to take responsibility for their own actions and think they can finger-point and sue whenever something goes wrong.
“If an AWD car comes to me with a failed tire, I will install only one new tire only if the tread difference is less than 3/32”. If the tread difference would be more than 3/32" it gets 4 tires"
Sounds to me like a 3’ vernier caliper might come in handy there!