Proper inflation of non-standard tires

Discussion in 'General Motoring' started by Ethan, Feb 14, 2006.

  1. Ethan

    Ethan Guest

    I have a 1996 SW1, 150K. At 121K, I had 4 new tires installed, not the
    original P175/70R14 size, but the P185/65R14 size, as I was told the
    original size was "rare" and was unavailable at the tire shops I shopped at
    the time, and that this other size was a suitable substitute. These newest
    tires have an 80K treadware rating, but I can see now I'll be lucky if I
    make it halfway there. After only 29K of driving, they're pretty bald.

    Here's my question: Is the proper inflation for these tires the same as
    that for the original size? In other words, is it the same 26psi rear and
    30psi front as the sticker on the inside of the driver's door indicates?
    I've kept the tires at that inflation, and rotated them every 6K, but, as I
    say, they're balding, and perhaps a little moreso on the center of the tire,
    which would suggest overinflation, right?
    Ethan, Feb 14, 2006
  2. ....not so uncommon (tire size) on I ran oversize tires also
    and ran the pressure slightly higher for occasional spirited driving.
    What's the UTQG (Uniform Tire Quality Grade) rating of your existing tires?

    Here's an interesting link related to wear ratings and the UTQG:
    Jonnie Santos, Feb 14, 2006
  3. Ethan

    Steve Guest

    Hi, Ethan,
    While I am no expert on tires, I certainly would not rely (for long) on
    the sticker placed on the car. The tire dealer or manufacturer would seem to
    me to be better sources for that information.
    Good luck!
    Steve, Feb 14, 2006
  4. Ethan

    Ethan Guest

    I checked out the link you gave and looked around the website a bit. I have
    the Yokohama Avid Touring. The UTQG is 620 A B. I also skimmed through the
    owner reviews. It seems that a few of the owners--especially the ones who
    have driven over 30K on their tires--have experienced short tread life, same
    as I.
    Ethan, Feb 14, 2006
  5. site I think - the last set of Bridgestone's I had were around 200
    treadware. Stuck like glue and wore like butter...
    Jonnie Santos, Feb 15, 2006
  6. expert here either. IIRC, the sticker on the car has tire pressure
    numbers based on what the engineers calculated the right pressure was for
    those tires and the specific car. I'm sure they probably have a fairly
    complex calcuation of what types of ratings/charteristics they want in a
    tire first, then they shop, sometimes they have a tire specifically made for
    them, then they get the tires, test them, play with the numbers and
    somewhere in the long journey they give us the sticker with the right
    numbres when the car hits production...

    The sticker has always been ground zero for me to start with.

    The numbers cast into the side of the tire are max pressure that tire will
    handle based on the engineers at the tire manufacturer, not a recommended
    pressure rating.

    Many cars have different air pressure ratings from the front to the back -
    since most tires (other than racing slicks) are not specific for the front
    or rear, there's no way the tire manufacture could include the correct air
    pressure for all cars.

    I'm guessing some of the guys that race probably use pyrometers to check
    tire temps when adjusting chassis setting and tire pressures, and could care
    less what the sticker on the car says...
    Jonnie Santos, Feb 15, 2006
  7. Please, oh please, unless you are racing and now what dimension your camber
    is set at, just leave the tire pressure set at what it says on the

    They generate those numbers universally, regardless of change in size or
    manufacturer. If it says inflate to 30 psi, please just inflate it to 30
    psi. I see severly overinflated tires at work everyday, so close to
    overfull they should have exploded..... but that is another story.....
    TheLastDonSC2, Feb 15, 2006
  8. Ethan

    Ethan Guest

    Thanks for the advice. Yeah, I surfed the articles on, as
    recommended by Jonnie. They have a lot of useful info on tires. I like how
    they specifically say that the "max" psi stamped on the sidewall is max the
    tire can be operated with, and not the proper pressure.

    Tire info has been elusive and contradictory to me over the years. As a
    govt. employee, I drive a lot of different vehicles. Typical scenario has
    been as follows: Another employee turns a vehicle over to me. Tires are
    visibly uneven in pressure, meaning that something is quite wrong. I look
    up "tire pressure" in the owner's manual. The chapter tells me, "Always
    inflate to proper pressure," but doesn't tell what that pressure is.
    Finally, after much jumping back and forth through the manual, I read that
    I'm supposed to check the placard on the door to determine the proper
    pressure. I check the door, and the placard is gone! Then I go to a
    coworker and say, "Well, dude, whaddya think the proper pressure should be?"
    And he or she responds, "What do you mean?! The proper tire pressure is
    stamped right on the side of the tire!"--like I'm an idiot. I tell you,
    the vast majority of drivers believe that tires should be inflated to the
    "max" pressure--if they ever think about their tires at all, that is. I've
    had so many people tell me this I almost started believing it.

    The article says that starting with 2003, owner's manauls are
    required to provide more info about tires. Maybe now we're not out of luck
    when the placard comes off. Incidentally, it is also mentioned that the
    manuals must now state the combined maximum weight of cargo and occupants
    the vehicle can handle. Jeez, it's about time. All the manuals I've seen
    just mention the combined weight of car, occupants, and cargo. Like you
    nothing better to do than look up your car's weight, then do some addition.

    Oh--and one other thing I forgot to throw into the equasion--what if you
    have two different tire gages and each gives a reading 6 psi different then
    the other? :) I guess there's a limit to how perfect we can be when it
    comes to such things as tire pressure.

    Ethan, Feb 16, 2006
  9. Ethan

    Bob Shuman Guest

    Do you want comfortable ride or maximum mileage? The door placard usually
    is the manufacturer's recommendation that provides a compromise with the OEM

    Me, I usually inflate tires that have maximum 44PSI cold pressure stamped
    into them to 35PSI for the given temperature (add air in cold winter months,
    take some out in hot summer months). On my tires that state 35PSI max cold
    pressure, similarly I inflate to 32PSI. This gives much better mileage and
    seems to increase tire wear as well. The tradeoff is a bit harsher ride and
    more wear on front suspension components.

    Bob Shuman, Feb 16, 2006
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