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Different size tires on all wheel drive

My brother in law has a 95' Subaru legacy.
<br/> The tire place put two new tires on, but they're a different size than what's on the front. He drove it for about 2 months. Now his mechanic says its stuck in all wheel drive, because of the different size tires. My question is.. Did he ruin his car? Or will putting the correct size tires on fix the problem?


  • edited March 2010
    If it's now 'stuck in all wheel drive', something's acting broken?, so he needs to get a complete set of matched tires (not old/new mixed) and get the drive train fixed. I would also press the 'tire place' to help with $$ (tires, at least), they should have known better.
  • edited March 2010
    The car is ALWAYS in AWD is it not?? Is there some kind of drivability problem?
  • edited March 2010
    Take a tape measure and go around the circumference(around tire) of front tire set and rear tire set. If the difference is greater than 1/2" it definitely damaged the AWD. How much is another question.

  • edited March 2010
    He says its hard to steer on pavement. If you drive over gravel, you can see all tires spinning. Like they're engaged. I haven't had the chance to measure the tires. If they are more than a 1/2" difference and the AWD is damaged. Is this something that I could fix myself? Or is this a specialized fix?

  • edited March 2010

    It sounds like the viscous coupler/variable transfer clutch is "locked up".
    Why/how did this damage take place? By using tires of different sizes on the car.

    The tire shop set up this sitation, due to their apparent ignorance of the importance of this issue on an AWD vehicle. The owner could have learned about it by reading the Owner's Manual.

    Probably the only fix is to replace the viscous coupler/variable transfer clutch (I don't know which one is on this particular model), to the tune of...maybe about $600. However, the first step is to put 4 tires of the same size on the car.
  • edited March 2010
    What kind of transmission on the car? An automatic has a solenoid clutch pack at the end of automatic transmission. A manual transmission uses a viscous coupling for AWD. Both require lots of labor. Manual transmissions AWD are much more tolerant to mismatched tires.

    First step though is buying four matched tires in size/brand/model/wear. Even if same size tire listed on sidewall again tires difference in circumference around.

    Last thing if this is automatic you can place an AWD defeat fuse in. Basically will revert the car to FWD and typically used when running spare. Not intended for extended periods but definitely at this point it may not be worth fixing AWD(if broken) on a 15 year old Subaru.
  • edited March 2010
    The use of different size tires on an AWD will cause problems. Usually sooner than later.
  • edited March 2010
    Your brother has damaged the AWD system on his Legacy by driving on mis-matched tires. Depending on the transmission type there are different fixes for this. Installing the correct size tires is not one of them, although he should do this anyway.

    If it's a manual, tell him to drain and replace the transmission and differential lubricants. If he's lucky it might help.

    If it's an automatic, he's probably going to need a new transfer valve and clutch pack. These components are located in the rear of the transmission. It cost me about $750 when I needed this for my '96 Legacy. The previous owner had been driving on mis-matched tires, and I didn't know it would damage the car.

    I know it now, and I make sure all four tires are evenly matched and rotated on a regular basis.

    If you want a cheap, temporary fix, insert a fuse in the AWD receptacle under the hood. It will disable the rear drive and the car won't be binding on turns. This only works on the automatic cars, and of course it will no longer have AWD, just FWD, but it will prevent internal transmission damage, which is what will happen if someone continues to drive the car the way it is.
  • edited March 2010
    Thanks so much for all the help. The car is a automatic. I think I'll try the fuse
    trick. Would it be okay to leave this in permanently? I'm not to worried about using the awd anyway.
  • edited March 2010
    This is an odd story indeed. usually all we hear is someone complaining the tire store is trying to rip them off because they need 1 tire on their AWD vehicle and the shop wants them to get all 4 new ones.
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