By on April 11, 2011

John writes:

Just a few days ago one of four nearly new tires developed a bubble on the sidewall. Thankfully, I purchased the roadside-whatever-the-heck when I bought them and got the replacement for the cost of shipping and had it mounted with decent haste – potential NJ turnpike crisis averted.

Now, I figure the other tires are at around 85-90% when this episode started. Is there a way to get the new tire to catch up with the others in terms of wear? Or should I leave well enough alone?

Sajeev answers:

The short answer is to leave well enough alone, it’ll be fine.  Well, that depends on if your vehicle has permanent, full time AWD?

John answers:

Nope. FWD.

Sajeev concludes:

Well then!  It’s not a problem, mount the new tire on the front axle and let it wear to match.  At your tread depth, that tire could be mounted at any location, even on a four-wheel-drive vehicle. The folks at Tire Rack go into further detail than I’d prefer to in this column, including shaving a street (not race) tire to keep someone from replacing all four tires when only one is truly bad. So all roads point to you being fine.

Which begs a few questions: how many people rotate tires as per owner’s manual requirements? As AWD becomes more prevalent in affordable CUVs and sedans, are we gonna see more problems with mismatched tires? If so, what mechanical failures should we be on the lookout for?

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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32 Comments on “Piston Slap: A Cautionary Tread Wear Tale...”

  • avatar

    I don’t think lightly mismatched tires will be a problem on the new bread of inexpensive AWD vehicles as most of them operate in an open diff FWD mode until slip is detected and only then do they send power to the rear wheels. Even then, most have open front and rear diffs, using only brake application supplied by the traction control to move power around.

    • 0 avatar

      The problem with mismatched tires and AWD systems occurs precisely when you have a system that only activates its second axle when there is slip detected, as the mismatched tire’s different circumference is read as slippage. Then the automatic AWD system tries to control said slip by engaging AWD and then by brake activated wheel speed modulation until something cooks or breaks. I’ve seen 3/32nds of treadware variation front to rear kill Haldex AWD systems more than once.

      • 0 avatar
        SVX pearlie

        And that is why some people ignore Haldex “AWD” and only consider Symmetrical AWD and Quattro to be valid choices.

      • 0 avatar

        You’ve still got to be careful. Some Subarus use viscous couplings to manage slip front to rear. The only bullet proof quattro system was the original, which had manually locked center and rear differentials. When I was in the business, we probably saw more Outback wagons with tire related drivertrain failures than anything else, but they were also the most popular AWD cars in my home town.

    • 0 avatar

      A true AWD system with a center diff it won’t mess with too much. However most modern “awd” cars are truly electronic 4wd and they can be messed with if the tire diameters are too far off. Many “part time” 4wd systems can also be messed with when they only have an “automatic 4wd” mode on the switch and not a “2wd” mode, like recent Explorers and Escapes.

  • avatar

    I tried that once with a Civic, at about 1200 miles, front axle, and it actually changed the handling. Putting the front tires, one new, on the rear, and the matched ones on the front cured it. Mike, the tire guy, just scratched his head.

  • avatar

    Might also be a problem if the car has a LSD.  I had a car with FWD and a limited slip and the slight mismatch in the tire diameter played heck with keeping the car tracking straight under acceleration.

  • avatar

    I’d had a similar issue with more worn tires once. I’d say that the tires were more like 75% tread left when I ran over a shard of metal. The road hazard warranty kicked in and I got one new tire. I noticed after a few days, that the car seemed to accelerate and steer somewhat differently, but was cured when I rotated the tires. From then on, I just lived with it, it wasn’t that unsettling. But if I’m in the same situation again, I’ll replace both tires on the axle.

    EDIT: I should note this was on a FWD car without an LSD. But it did have equalized half shafts. I don’t know if those would make a difference.

  • avatar

    That photo reminds me of the hernia that I had an operation on…

    • 0 avatar

      Yeah, Me too. I thought the very same thing! Nothing else to add here, except from the above info, it doesn’t matter what axle it goes on – a non-issue.

  • avatar

    With some AWD systems, such as Subaru’s and Volvos through 2003 or so, it’s absolutely necessary to have four tires of the same size.

    I have some tires from CostCo, and this has made it very hard to properly rotate the tires. They can rarely rotate tires in less than an hour, so I avoid getting it done. And if the tires different in size by more than a few mm they refuse to rotate them, as they insist that those with the most tread be on the back. With a FWD car this is a recipe for always buying tires in pairs and missing out on sizable coupons.

    • 0 avatar

      @Michael: I’ve never heard that about Costco, but I’ve never been able to take advantage of buying tires there. My cars seem to have sizes they don’t carry or if they do, there’s only one make of tire to consider. I’ve done much better with Discount Tire and Belle Tire stores. With those two I usually can get “good-better-best” choices of tires, and the prices are competitive with anything Costco carries. Both Belle and Discount offer free rotations, they work you in as soon as they can, and neither of the stores I’ve dealt with have tried to ‘upsell’ me on anything (while waiting for rotations). I can say that I’ve had very good experiences with both companies.

      • 0 avatar

        Discount Tire where I live gives the best service of any company I have ever dealt with. Before I bought tires from them, they repaired a number of flats for my friends and I for free. When a girl I lived with drove on a donut that was underinflated and killed it, they gave her one that they had laying around for free. When I was ready to buy tires, I wanted to buy from Discount Tires. Their prices weren’t great for the tires I wanted, but they price match. I wound up getting the top rated tires on the market for the Tire Rack price from my neighborhood store that will fix flats, rotate my tires, and even check tire pressures if I stop by. From what I’ve seen at CostCo, they basically offer a few choices in most sizes. I’m sure some of their tires are acceptable, but the odds of getting state of the art tires perfectly suited to how you use your car are few. Tires are the single biggest determinant of how a car rides and handles. Minivan tires on a Ferrari will give you a Ferrari that steers and stops like a minivan. Ferrari tires on a minivan will give you steering feel and the ability to lift the inside rear wheel on off ramps. You can tell whether or not someone knows about cars by their tires.

  • avatar

    Yes, I rotate per the manual. Anyone who has owned an older Ford truck or SUV with Twin-I-Beam front suspension knows how important it is. While the Tundra has decent front suspension geometry, I still rotate. The car is FWD so I want all the tires to wear out at the same time, so it gets rotations.

  • avatar

    I had a year-old Michelin Pilot Sport AS tire develop a lump after a pothole hit on my ’04 Volvo S60R. I took the car to Costco to see what they could do for me since that’s where the tires were purchased. They said they would have to replace all 4 because of the AWD. With their road-hazard warranty and promotional discount on new Michelins, my net cost for a full set was $35. Plus, free Nitrogen as usual with them.

    • 0 avatar

      Wow, now thats a good deal, I am surprised the road hazard covered more than just the single damaged tire!

    • 0 avatar

      That’s great that they replaced all 4, quite surprising. However do yourself a favor and let the Nitrogen out and get good old plain air in there.
      While it sounds good in theory, leaks less, and more stable pressure vs temperature the car mfg decided on the optimum pressure based on air being in the tires and took into account that the tire pressure will increase with tire temp. That is a good thing as that helps to control overheating. The reason a tire heats up is because it flexes as it rolls, the higher the pressure the less it flexes.
      You’ll notice in most owners manuals they tell you to set the tire pressure cold and to expect it to increase with driving (particularly high speed driving) and not to bleed out any pressure increase due to increase in tire temp.

  • avatar

    Nah, I’m thinking a weekend with a piece of really gritty sandpaper!


  • avatar

    If you swap winters and summers, that’s a convenient time to rotate. Just make sure you mark which tire was where before you store them for the off-season.

  • avatar

    I started doing my own tire rotation, when I realized that my tire realer wasn’t following vehicle manufacturer’s instructiions. They were doing a simple front to rear swap, whereas the manual called for the rears to cross-over as they moved to the front, and the fronts to go straight back (when the spare wasn’t included in the rotation). I don’t know how critical this is, but on a full-time four wheel drive, I’d rather not find out the hard way.

  • avatar

    My dad, being the frugal guy he was, used to only buy 2 tires at a time when my 1993 Impreza needed new tires.  Never saw any major negative effects of the different diameter front to rear.  I don’t believe that it had a limited slip center diff, though.  Definitely not something I’d do with a newer, nicer Subaru, though.  This MY93 was at 90k miles and it was on the first of 3 teenage drivers.  Things weren’t going to end well for that car regardless of the tires he put on it. 

  • avatar

    Mismatched tires were a significant cause of problems in our 2000 Volvo V70XC.  Twice we had CV problems, and propeller shaft problems.   We worked hard to keep tires rotated and of a similar circumference, even replacing them all when one was damaged with more than 50% tread life left.  But the mechanical system in that Volvo had no ability to adjust.
    I was advised that the subsequent Haldex system would not react the same way and wouldn’t treat similar differences in circumference is slip.  I suspect that the modern systems, whether Haldex, Subaru, Quatrro or X-drive etc. can accommodate some variation more easily.

  • avatar

    This suggests that some of those AWD and Limited slip dif cars should require real spares.
    On the other note, we got a bubble in the tire crossing the country in the old Studebaker back in ’57.
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    • 0 avatar

      In case of Subaru’s they don’t have a full size spare (at least my 2006 Legacy doesn’t).  On the automatic transmission models which are electronic, you must insert a fuse to disable the AWD and make the car FWD if you need to drive more than a few miles.  Another reason people need to read their manuals.

      Subaru also requires that all 4 tires not differ more that 1/4″ in circumfrence.

  • avatar

     I rotate front to back every 15000 KLM, on my Impala. I shudder at the thought of replacing those 18″ tires. I’m trying to get as far as I safely can, with the originals.

    I used to do the same with my Jimmy. However the wicked cost of gas {almost $5 USD} finds the Jimmy not moving too far. I’m more concerned with flat spots on my pricey Michelins.

    As far as “johns” question goes, 10 to 15 percent difference? Don’t sweat it. If it was 30+ percent buy another tire to match the axle.

    • 0 avatar

      Mikey, you might have the best luck asking your local dealers or checking ebay/craigslist for “takeoffs”.  Frequently you can get a basically new set of tires AND wheels for the same or less cost than just the tires would have cost by themselves. 

  • avatar

    My car manual specifies 1/8 inch max tread height difference. I rotate every 5000 miles .
    The problem is that many tire stores have a zero tolerance for “any” tread difference on an AWD car. They use this a marketing opportunity to have you replace all four, “it’s not safe”  unless you have the road hazzard warranty and then it’s not so critical.
    I damaged a sidewall on one of my almost new Michelin Pilots (Car purchased used with them already installed so no warranty) .  I ended finding a pair of matching tires with identical tread depth on E bay and taking the single wheel to the tire store to have it switched.
    I wonder how important this all is, since many people don’t rotate and front tires wear out much faster any way. It also seems to me that depending on how many people are in the car the circumference of tire would change.

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