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  1. #1
    Join Date
    Dec 2004
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    Wisconsin
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    What distance can one drive an AWD with the spare tire?

    A dealer technician thinks one can drive about 50 miles on a spare, but wasn't certain and didn’t have a source for that information. Can anyone confirm that?

    The spare tire is one-inch shorter in diameter than our glorious Continentals. That’s roughly 3.5% change in circumference.

    The owner’s manual, page 235, has a strong cautionary statement that unequal tire size may cause failure of the front differential and/or the transfer case. It is curious that the rear differential is not mentioned, but we will leave that aside for the moment.

    The manual indicates that the spare is for “temporary emergency use” and to limit the speed to 60 mph due the spare tire’s adverse effect on handling. No mention of how many miles can be driven before the smaller diameter spare might cause drive train problems. Since there is no distance limitation provided, it leaves me wondering what distance might qualify as emergency use?

    I could envision a circumstance of traveling in a remote area late at night, destroy a tire and need to drive 100s of miles on the spare because no replacement tire is available.

    Since I drive in remote areas at night about once a month, the chance of problem is not high, but it seems possible. I have not needed to use a spare for many years, because I carry a patch kit and compressor, so I can avoid being screwed by the typical screw in the road.

    The situation is worsened if slightly (1/2”) larger than stock tires, e.g. 235/60 or 255/55 are installed. However, the difference compared to stock size is reduced to virtually zero once about 1/3 tread has worn away.

    If the potential for a problem remains murky, it may be possible to carry a full sized spare by removing the rear sub-floor… I’m going to check that.

    I’d appreciate some input. Thanks in advance.

    Scott
    Last edited by Scott; 11-21-2005 at 03:12 AM.



  2. #2
    Join Date
    Sep 2004
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    I would go for the "5th wheel" idea... Make sure you have it in the rotation pattern so it wears evenly with the rest of the tires and it will offer you the ultimate in peace of mind which it looks like this is what you are seeking.


  3. #3
    Join Date
    Jul 2005
    Location
    Lexington, KY
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    Scott:

    You have brought up a point of great debate! When I had my tire shop we ran into this AWD issue all the time. If we sold someone a tire that did not match up to the tires on the car and the drivetrain was damaged then we were on the hook for the repair bill.

    As a rule of thumb, we would only mix tires that were within 1/4" of the circumference of the tires on the vehicle. There were a lot of Subaru's where we were. I know if you mixed tires on those it automatically voided the drivetrain warranty.

    I always wondered how that could be because the temporary spares are never even close to the same size as the full size tires.

    I can't imagine that driving any amount of mileage on a temporary spare is good for the drivetrain, but if you get a fullsize spare and try to use it after your original tires are worn down 50% then your in the same boat.

    I think I would ask for something in writing from Dodge concerning the issue.

    That's my 2 cents worth. Hope it helped.

  4. #4
    Join Date
    Dec 2004
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    Wisconsin
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    Fred & deuceroadster2, thanks for replies:

    Fred: A 5 wheel rotation pattern would provide a ready back up for a destroyed tire, but it would also mean nearly always running the car with slightly uneven tires and possibility stressing the drive train continually.

    For instance, let’s assume a tire rotation every 5000 miles and a tire life of 25 thou miles with replacement at 5/32” tread loss. The 5th tire will have 1/32” more tread than the other tires until the 4th rotation at 20 thou miles. 1/32” difference in tread depth between tires corresponds to about 2 tire revolutions per mile of slippage that will have to occur in the drivetrain or tires.

    It comes down to not knowing if there is threshold, below which, a difference in tire size causes zero short or long-term problems. If there is a few tire revolutions per mile difference between front and rear wheels, then where does the slippage/strain occur?

    It might be helpful if I understood how our AWD system is designed. My copy of the service manual (free from a link on the web) does not provide any details on the AWD or explain how it works. Fred, in another thread, you wrote that the 38/62 power split is accomplished with planetary gears, but are there also some clutches something that slips in the system? Is the chapter of the service manual that provides details on the transfer case available?

    deuceroadster2,
    “I can't imagine that driving any amount of mileage on a temporary spare is good for the drivetrain, but if you get a full-size spare and try to use it after your original tires are worn down 50% then your in the same boat.”

    I feel fortunate to receive a reply from someone that has been down this road before and sees the many aspects to this issue. I think you are right-on, any difference is probably not good. However, wouldn’t you think that one is better off and could travel further with a full sized spare because the difference between tires is much closer than with a small spare?

    Tire shop nightmare: One must sell a lot of tires to make up for one Subaru AWD repair! Do shops have insurance for that? What is the smallest difference in tire size that you remember causing a problem? How did you arrive at the 1/4 inch spec for circumference? Did you ever inquire with Subaru?

    Another question: Can someone confirm that an open differential transmits the average speed of the two wheels to the drive shaft? (Ignoring the axle ratio.) If this is correct, then the difference at the transfer case may be 1/2 the tire speed difference.

    Thanks for the suggestion to request a written statement from Dodge. May be a frequent 5 tire rotation may work? I will also check for owners forums for Audi and MB 4-matic. They have been dealing with this issue for 10+ years, so they ought to know some good approaches.

    Thanks guys!

    Scott

  5. #5
    Join Date
    Jul 2005
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    Scott:

    We never had a problem because we flat out refused to mix tires on AWD vehicles. We also had a "waiver" that one could sign if they thought we were giving them a line of bull*&#@. The waiver basically said that mixing tires WILL damage your drivetrain and it was explained and they understand it and we will not pay for any such damage.

    The waiver came about due to some folks accusing us of trying to make them buy four tires. Some folks just don't get it.

    We actually got our information right from the Subaru dealer, as far as tolerance goes. I have forgotten now if they recommended 1/8" or 1/4" tolerance. It seems that they may have been at 1/8" and Audi was at 1/4". I remember Audi's tolerance being more than Subaru's but I'm not sure why.

    And, yes, our garage insurance would have covered any damage that could have been caused, but we would have fought it due to the waiver.

    I think the 5 tire rotation would be the best option. I still don't see how they can give you such a small spare tire (donut) and then tell you if you mix tires you will void the warranty. It seems contradict itself.

    Good Luck.

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