By on February 13, 2012


John writes:

Hey Sajeev,

Since you requested goofy questions for Piston Slap, I’ve got one:

A friend of mine told me that her dad had a Subaru (I don’t know what particular model). He had one of the tires blow out, and even though he knew he should either replace all four tires, or have the new one shaved down to match the other three, he decided to risk it and just use the one new tire as is. Consequently, the all wheel drive system got messed up due to the ever so slight mismatch in tire diameters. Is this for real? I’ve never heard anything like that before.

Sajeev answers:

I didn’t request “goofy” questions, per se…but let’s not split hairs. And this is far from a goofy question.

Because this problem is for real, a good explanation is here. And it’s not an “ever so slight” mismatch with the tires, if the ¼” circumference discussed on is valid. I regularly dissuade people from buying AWD cars, unless they live in colder climates where municipalities simply can’t regularly plow all their streets.  Which is a lot of the country, but not a lot of the population.

AWD systems are heavier, thirstier and cost more to buy. For the long term owner, they cost more to keep functional. And for anyone who loses one tire from a quartet that had a lot of life beforehand, things get real ugly. This is almost as silly as fretting over horsepower figures when wide-open throttle is rarely applied.

I will try to remember this letter the next time someone asks my opinion on an AWD vehicle, when I know that someone doesn’t need it.  And that’s not a slam on Subaru or Audi, at least not intentionally. The concept of shaving down a perfectly good tire will certainly get some “financial traction” in people’s minds!

Send your queries to [email protected]. Spare no details and ask for a speedy resolution if you’re in a hurry.

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47 Comments on “Piston Slap: Financial Traction, AWD Distraction?...”

  • avatar

    Yep, it’s true. Even a slightly different sized tire will cause harm to most Subaru AWD systems. Older Subaru’s that had part time 4WD, like the old GLs and the like you could probably get away with running a slightly larger tire in the rear since it would only be used once in a while, but the other full time AWD systems will burn out when one wheel is spinning faster than others.

    AWD systems are slightly more expensive than a 2WD system to maintain, but it’s totally worth it if you grew up driving 4WD or AWD vehicles. My problem is that having an AWD Subaru actually makes my commute to work considerably longer as I’m always on the prowl for an un-plowed road or street to bust through.

    As for the tire scenario, just sell the other 3 good ones on craigslist.

    • 0 avatar

      Been there, done that, have the receipts…

      Even replacing two tires in a Subaru is a no-no. We did it on my wife’s ’05 Outback and it developed some characteristics…even some noise…that was only fixed by going back and replacing the other two tires before the first two got too worn down.

      Problem solved and we learned our lesson. Replace one tire on a current Subaru symmetrical AWD, replace ’em all.

      • 0 avatar

        hmmmm… gee, this sounds like another Urban Myth… like alligators in the New York sewers… I say this, because we replaced *ONE* rear tire on my wife’s 1999 Subaru Impreza… and, 100,000 miles later, the result? Nada. Nothing. No damage. No weirdness. Nothing. Is Honda or Toyota paying folks to make this stuff up? (attention PR folks at Nissan: you guys need to catch up the other guys, and contribute to all the fake negative complaint postings against Subaru!)

  • avatar

    I used to live in one of those colder climates with lots of snow, and this was one of the (many) reasons why part-time 4WD systems were pretty popular even as full-time AWD started to be available.

    Nowadays, though, they’re getting rarer than manual transmissions.

  • avatar

    With wifey’s 2002 CR-V coming up to its 10th year, the possibility of having the AWD going out or getting sick really worries me. May get rid of it when that time comes. So far though, no issues.

    • 0 avatar

      No worries mate. Your wife’s CRV never really had an AWD system in the first place. It’s more like an AWD lite system that sort of engages the rear wheels as the front tires start to slip.

      Despite the Honda’s lack of 4 wheel grip compared to others, it has proven to be a reliable system. Just keep your fluids changed per Honda’s recommendation and you’ll be just fine.

    • 0 avatar

      are you serious? That is the most reliable system in the world. Stop worrying!

    • 0 avatar

      Our 2002 CRV has 197k miles and still working perfectly, running strong, pretty much like new. From what I understand, just keep the diff fluid changed out as recommended and you are fine…

  • avatar
    Educator(of teachers)Dan

    I too favor part time systems for this reason and for the fact that then you’re only truly using the sytem when you need it. I know I had to be extra careful with the one vehicle I owned with posi-trac because tire diameters that are off can ruin a tight posi too.

    Sajeev, wouldn’t this be true for certain traction control systems too? Could tire sizes being off upset the computer?

    • 0 avatar

      Yes, part time systems are simpler most of the time then AWD, however, the problem with part time 4WD is, when you needed it, it will not be engaged autonomously, therefore no real benefit. Example someone hit patch of ice, with AWD, it will kicked in instantaneously, most of the time, with part time, not so much. Also think about it, when your better/worst half driving it, will she be able to know what to do? Have witness too many cases when part time 4WD SUV was spinning real wheels because the driver “forgot” to engage 4WD.

      • 0 avatar

        4WD isn’t going to do a hoot of good when you hit a sheet of ice. If you have any tires with traction, they will still have traction even if they don’t have power going to them. If you have zero tires with any traction… well, you don’t have traction and 10WD won’t help you. You’re best served by not making any rash stabs to the gas or brake or jerk the wheel but rather try to smoothly navigate through the ice patch.

        4WD is great for getting you moving but it does little when it comes to turning and stopping. (Unless you commute through snow like Sebastian Loeb with all 4 spinning.)

        I explained it to my wife. Is there ice and snow on the road? If yes, put it in 4WD and keep your speed in check. If no, 2WD is just fine.

    • 0 avatar

      Dan, not 100% sure, but I can guess. Since traction control is purely electronic/brake (no differential involved) and it usually kicks in only when one wheel spins much faster (relative to a circumference issue) than the other, I can see this not being a problem. This was a problem when I drove a car with non-stock wheel sizes in the rear (Corvette Z06 with race tires) and the traction control did have to be de-activated. The computer thought the car was constantly doing a burnout!

      Not sure how big a difference is needed to mess up a posi-trac on a RWD-FWD vehicle. I will leave that to the B&B so I can get back to my day job.

    • 0 avatar

      Yes, mismatched tire diameters can throw off ABS and TC. This is a frequent issue for pony car owners when the slap some massive tires on the back without taking in consideration the overall diameter – the constant difference in rotation speed makes the computer think there’s a problem with the system so it disables ABS and anything else that depends on it. Side to side mismatches should have the same effect.

      I think the percentage they can be off is pretty generous though – my car is 3% I believe and with its 24.5″ tall tires would, that allow almost 3/4″ difference in tire diameters, or a 3/8″ difference in tread.

  • avatar

    So now I feel stupid for buying a Subaru in a climate where snow is just a 1/2″ thick layer of slush that appears once a year to give kids an excuse to not get up for school. Thanks…

  • avatar

    AWD is basically an ego-soother for less talented drivers. I remember a co-worker who said he loved it because he could go flying down the freeway in the snow covered lane. Then I reminded him that the laws of physics must be obeyed when you try to stop, AWD or not.

    I’ve lived my entire life in cold snowy climates and have never had AWD vehicles. Stuck only once or twice when I didn’t survey the road ahead and went into larger snow accumulations. Anti-Lock Brakes and Traction Control are more useful to me and don’t add extra weight/cost like AWD does.

    • 0 avatar

      All cars have AWB (all wheel brakes) and are subject to the same unyielding laws of physics and friction for stopping as an AWD vehicle. Many AWD vehicles are heavier and take longer distances to stop. Just because you can get going an unsafe speed (with AWD) doesn’t mean that it’s advisable for good control.

    • 0 avatar

      I don’t think calling it an “ego-soother for less talented drivers” is fair. I know several people who live in ski country WV who probably couldn’t make it up their driveway without AWD and snow tires.

      AWD is a tool that offers some good advantages along with some extra (and possibly costly) precautions. Just figure out your ROI, TCO, and see if it’s right for you.

      Personally, I’ve not been in a situation where I needed AWD. FWD and snow tires have yet to let me down.

    • 0 avatar

      Less talented drivers, or maybe people who live where there are snowy hills to climb. Or with steep driveways. Or who want to drive on the beach.

      People talk about the weight of AWD as if it really matters on a commuter car, and about the vast abundance of traction available as if it were just an invitation to trouble.

      It’s traction. Is more really a bad thing? What if you know how to drive but just want more traction?

      • 0 avatar

        “maybe people who live where there are snowy hills to climb. Or with steep driveways.”

        That’s why a 50/50 balanced RWD car is better than FWD in snow. I live on a steep hill and have both FWD and RWD cars. The RWD cars are significantly better climbing when the road is unplowed.

        On trick to remember with steep driveways and FWD is to simply back up the driveway.

      • 0 avatar

        Must be some steep hills, mcs! Assuming a fairly typical 22″ center of gravity height and a 110″ wheelbase, you’d need a 25% grade for a 50/50 RWD vehicle to have as much weight over its drive wheels – 55% – as a FWD vehicle with 60/40 weight distribution in the same situation.

    • 0 avatar

      It’s too bad you’ve never had the opportunity to try something as fun as this:

      EDIT: Not sure where the Youtube link is.

      Search youtube for this video zIDe6kW7GeA

  • avatar

    Subarus are like this, but so are the millions of pickups, SUVs, crossovers, etc. that have some variant of 4WD or AWD. Some dealers even replace wheels and tires on 4×4 SUVs but don’t replace the spare so you see dumbasses shredding their diffs on the highway with a small spare on a 4WD Denali. How much any of these cars can tolerate is anyone’s guess. My Land Cruiser doesn’t seem to mind mismatched tread depth on the back tires, but it’s also older than 1/3 of the drivers on the road these days.

  • avatar

    Agreed…this is a real deal. My mother-in-law got a brand new 1999 Legacy SUS when she was a home-health nurse whose patients needed her to show up regardless of weather. She hated the car, and handed it down to my wife as soon as she no longer needed it. My wife and I now own the car, but while we were dating in college my wife’s brother borrowed the car to move out of his place in northwest New York state. During his trip he hit a pothole and blew out two tires. When we got the car back I noticed it making a clunking/thunking accelerating with the wheels turned. Thinking it a CV joint, I told my wife to take it to Subaru (they had an extended warranty) to have it checked. Turns out my bro-in-law only replaced the two tires that blew, put them both on the front, and in time this tore up the center differential. Luckily the repair was performed and covered under the extended warranty, and my father-in-law drove from Subaru to the tire place to have a brand new set installed.

  • avatar

    My experience has been that the old viscous coupling based mechanical systems were much more sensitive to this than the newer systems, like the current Subaru system or Haldex, X-Drive and Quattro.

    My old 2000 Volvo XC 70 chewed up a propeller shaft, VC and other expensive parts. And yes, later I had to dump 3 perfectly good tires when we blew one. Annoying as hell. Never had a problem with our Tribeca, and we haven’t yet with our Lexus RX or my 328xi. I need AWD just to get up my driveway in winter.

  • avatar

    If a slightly larger tire is enough to cause mechanical damage to a subi 4wd system, how do they handle the outer wheel having to turn faster than the inter wheel during turns? Does the same damage occur at those moments and it’s just a function of turning being such a small part of the life of a car that it doesn’t cause immediate damage?

    • 0 avatar

      That’s what the system is designed to handle — intermittently, and generally at low speeds. It’s not designed to deal with it nonstop, at freeway speeds. You don’t damage your brakes because you stop at red lights, but if you left your foot on the pedal all day you probably would.

    • 0 avatar

      That only affects the simple axle differentials, which do not use any viscous couplings (MT) or clutches (AT). It’s when the back wheels and front wheels are forced to turn at different speeds that the center differential can get destroyed; if it happens at high enough speeds, with enough speed differential, and for a long enough duration to cause damage.

  • avatar

    Had to laugh. A Subaru ad loaded next to the article for me. And an Audi SUV ad at the bottom of the page. Good to see TTAC integrity intact.

    Personally I think AWD is worth it in certain parts of the county.

  • avatar

    Um, search around a bit and buy a used tire. If you are still running on OEM tires (Yoko’s for Subaru’s) it’s pretty easy to find used to match your current tread depth of the other 3. I’ve replaced 2 tires in my Subaru this way over the last couple years.

    Oh, and it’s not a bad idea to have an extra used sitting around in case of a flat or something. Used can run $25-$75 while a new OEM is at least $125.

  • avatar
    Chicago Dude

    About 10 years ago, I owned a AWD vehicle (Audi) that came from the factory with 5 matching wheels and tires – the spare was the exact same as all the rest. It was strongly suggested that I follow a tire rotation pattern that included the spare tire and to do the rotations every few thousand miles.

    Does anyone still do this? I’m pretty sure that Audi has switched to the space-saver spare so they don’t have to give you 5 expensive alloy wheels when other people can only see 4 at a time.

    • 0 avatar

      I do this. When we got our 2006 V70, I got the full-sized spare. Even though the car is FWD, I do the rotation to keep the tread somewhat even. I also wanted the full-size spare so in case I did get a flat, I can continue a trip and get the new tire at my convenience, not within 50 miles like with the donut spare.

    • 0 avatar

      I have an 87 Audi 5000 Quattro.
      It has a switch on the dash to control locking the front & rear differentials separately via air lines.

      This is how it was explained to me…

      Running with the switches off I have power to both the front and rear differentials. Each differential acts as standard differential in the “off” mode. So in this “off” mode it can effectively turn corners, get better mileage, reduce tire wear, and reduce drivetrain stress. In this “off” mode you have in effect one wheel drive up front and one wheel drive out back.

      Turn the switch to the first position and it locks the front differential. Turn it to the second position and it also locks the rear differential.

      The locking modes should only be used when needed. Snow, gravel, extreme hills and the like.

      So tire size would only be critical in the locking modes when you can achieve full traction to the road. So I would imagine that Audi is asking you to rotate tires so you keep them in proper diameter if you happen to use the locking modes on dry pavement.

      The newer Audi’s use a Torsen locking system. I have not researched how that works.

      • 0 avatar
        Educator(of teachers)Dan

        @TrendShifter, I wish that Subaru still offered driver controlled locking diffs in their vehicles. I’d actually be excited to buy an Impreza hatch with manual trans and driver controlled diffs.

      • 0 avatar

        I had one just like it, (only it’s called Audi 100 here, and has different headlights). It works more or less exactly as you said, but I think that with three open diffs in ‘open’ position, it’s completely ‘random wheel drive’, and can theoretically end up spinning only one wheel. I live on top of a steep hill, and we have real winters, so the locking diffs came in useful.

  • avatar

    Our ’04 Forester blew one of its [email protected] OEM tires early on, so we replaced them with much better ones that have lasted 7 years of mostly suburban driving plus a few long-distance runs on the Interstate system. Tire rotation is only F-R, since the tread is directional and they must stay on the same side of the car. AWD durability has never been at issue. Same for our AWD Volvo, which has had higher mileage and a couple tire replacements (all 4).

  • avatar

    When my car was semi-new (2005 STi) I had a blowout on the front left tire. I replaced both front tires & left the rears the same. I always swap the front/rears at the same time (STi has a back to front tire rotation pattern) and haven’t had any issues. I’ve since replaced all 4 so am matching again.

    the AWD thing, honestly is a huge pita…although I was thankful for it during our last big snow & my subdivision didn’t get plowed for 3-4 days. Other people got stuck. I got home. It is unnecessary in Chicago.

  • avatar

    I put a roofing nail (don’t ask) through one of the tires on my Outback about two weeks after delivery, and changed out just the one tire–but this was still very early in the tire’s life. Much later in the life cycle of the tires, and I’d have had to replace all four. But on the other hand, it’s not as if flat tires are all that common; in 26 years of driving, I think I’ve had three.

  • avatar

    I strongly doubt this urban legend. The tiny bit of variation in tire diameter, is miniscule compared to the normal steering and the constant road surface variations that the tires see. The viscous center diff is just like another torque converter. A teeeny bit of slippage is nothing. Most AWD cars do not have any kind of limited slip differential in the front.

  • avatar

    Might be a Subaru thing?

    Toyota Alltrac systems also used a viscous center diff but having been on the Celica bulletin boards since 98 or so, I can’t recall ever hearing one problem with their VCD design. Alltrac system is split 50% front to rear fulltime.

    I have had to use the string method when buying a used tire once, as I always try to maintain same diameter. Just to get home though.

    It should be noted that different brand tires the same numerical size can have different diameters as well as different rolling resistance that can affect driveline.

    And in Colorado, steep driveways combined with unplowed conditions make AWD somewhat needed unless you enjoy walking. And many gravel roads makes for lots of fun. Its real advantage for cars like the Celica lies in cornering speed in the wet or dry. See also Audi during its IMSA takeover.

  • avatar

    The most common Subaru AWD system is the Multi-Plate Transfer clutch behind the 4 speed auto (4EAT) and the new CVT. It is always engaged when the car is in gear, and has a 60/40 front-rear balance statically. (90/10 went out for the 1998s.

    When the tires are all the same make and nominal size and the car is travelling in a straight line, the front and rear wheel speed is the same, and the MPT clutch plates do not slip relative to one another, so no wear happens. When going round curves, the front wheels travel a lesser distance than the rears (in any vehicle), so there will be slippage between the clutch plates to allow for the difference. This is a short-term phenomenon, of course.

    When one wheel is differently sized, the clutch has to slip all the time, wearing it. The same can be said if the two wheels on an axle are more worn than than the pair on the other axle. So some tolerance has to be incorporated to allow the MPT clutch system to have some longevity. Subaru says 1/4 inch on tire circumference, which amounts to only about 0.3 %. My opinion is that it is just to cover their asses. Also, remember, you can yank a fuse to make the car FWD only if you get a flat and have to use the spacesaver spare, because it’s NOT the same circumference by a long shot.

    At a 75 mph cruise, the typical tire size of 215/45 – 17 rotates at 1025 rpm, so the clutch plates scrape over each other at a mere three (3) rpm, if there is an overall 0.3% difference between front and rear tire circumferences. That’s one revolution every 20 seconds, or three times faster than a sweep second hand on an analog clock.

    If only one tire is replaced, then that axle turns only 0.15% slower than the other with the worn tires, and the differential on the axle with the new big tire takes up the difference between tires. No biggie.

    Subaru and the other AWD manufacturers know this tiny difference doesn’t amount to a hill of beans, but won’t come out and say what the real tolerance is. I’d bet it’s easily twice as much. The thing to remember is that different makes of tires with the same nominal size can be quite different in actual size and tread width, so get at least the same make and model if you have to replace one. I wouldn’t worry until the tread depth difference is at 3/32nds or more. Then shaving is called for. Put the new tire on the front to make it wear more quickly to the other old tires’ size.

    For manual tranny Subies, the center differential will look after the mismatched front and rear wheel axle speeds. The viscous limited slip device connected between the front and rear output shafts of the center diff won’t work up much of a sweat with a 5 or 10 rpm relative speed difference. 100 rpm or more, sure, that’s what it’s there for, to lock up.

    With the 5 speed auto in older Legacy GTs, 3.0 and 3.6 six cylinder vehicles, and early 4 speed WRX autos, there is a center differential, like the manual tranny cars. There is also a lockup clutch to lock the front and rear driveshafts together when conditions warrant. There is no fuse to pull to put the car into FWD as there is with the much more common MPT setup in an average Subie, because it isn’t needed due to the center differential.

    After some 2 1/2 winters and 9000 miles on my Legacy GT auto, my front right tire expired with a large nail through the upper sidewall. Unrepairable. A new Toyo winter tire was 3/32 deeper in tread, and the Subaru dealer installed it. On the front. I have had no troubles at all. Car tracks straight in the almost two winters since. Didn’t expect to, either. A reasonable amount of common sense has to be applied.

    I have only had AWD cars since 1988, and haven’t had to replace so much as a CV boot, so where all this extra maintenance is supposed to happen compared to FWD is beyond me. Having traction all the time is a true luxury, unlike leather seats or real wood inserts on the dash. I am constantly amazed at the poor traction of FWD when I travel in friends or colleagues cars in snowy conditions, in fact it’s white knuckle time for me.

    Sure, if you live in the sunny south, then two-wheel drive is fine, I guess. I prefer my real luxury to be on my side, no matter the weather.

    • 0 avatar

      Like you say there is a lot of CYA here. The tolerances on tires is just not that tight.

      If you load your car up with people or crap in the trunk the rolling circumference will change. If the tire pressures are slightly off the same thing occurs. If you drive cross country like that will that destroy your car?

      I have had several Audis, I believe the allowable depth difference is 1/16 inch. The biggest problem I find is the tire shops have a zero tolerance policy, except if you pre-purchased the road hazard warranty then they are more liberal about not needing to replace all of them.

    • 0 avatar

      “The thing to remember is that different makes of tires with the same nominal size can be quite different in actual size and tread width, so get at least the same make and model if you have to replace one.”

      Sometimes that’s not even enough! From a forum I freqent, here’s a situation that an actual tire engineer encountered when four new apparently-same tires were installed on some sort of AWD Cadillac:

      “Last week I got a call from a Cadillac dealer on a 2008 AWD. He had installed a new set of tires and within a week the guy was back with tranny noise. The tech diagnosed this as tires with different diameters – about 0.4″. Same size, same make, same model, same speed rating, but different plants. They called to ask about next steps.

      I arranged to have the tires shipped to me. They arrived today!

      Not only different plants but the date codes were only 8 weeks apart – AND – the earlier tires were made of rayon with a single nylon cap while the later tires had polyester and 2 cap plies.

      And I could see the difference without even mounting the tires up.

      Arrgh! $10K to fix everything!”

  • avatar

    I owned one AWD vehicle – a fourth-generation 4Runner. It was great, but it also cured me of any notion that AWD is remotely necessary anywhere south of I-70. The few times it could really come in handy don’t make up for the added expense.

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