Tire air pressure question

Discussion in 'General Motoring' started by tomkanpa, Jan 28, 2006.

  1. tomkanpa

    tomkanpa Guest

    The tires on my 2001 PT call for 34 psi cold tire pressure.
    Since mpg and tire wear are based on the amount or tire tread* that
    contacts the pavement, wouldn't it make sense for the front tires to
    have a higher psi to compensate for the weight of the engine?
    Looking at the tires makes it quite clear that more tire surface of the
    front tires contact the road than the rear tires.

    * I know that other things such as driving habits, tire sidewall flex,
    etc.. also effects the mpg and tread wear, but my question is just
    about the air pressure.
    tomkanpa, Jan 28, 2006
  2. tomkanpa

    Matt Whiting Guest

    Not necessarily. I'm not a tire engineer, so I don't know the answer to
    your question for sure, but here's something to think about. The
    question is whether it is more important to have the same pressure on
    the road from all of the tires or the same area on the road. I'm
    guessing it is better to have uniform pressure at each tire and thus the
    reason that the same pressure is specified for each tire. If you
    increase the tire pressure to make the contact patch size the same front
    and rear, then you are also increasing the pressure on the road exerted
    by the front tire as compared to the rear.

    I suspect having different contact pressures is probably worse for
    handling, etc., than is having different contact areas.

    If you know this, why do you state above that MPG and tread wear are
    based on the amount of tread that contacts the road?

    Matt Whiting, Jan 28, 2006
  3. tomkanpa

    Bill Putney Guest

    Lots of flexing and high rate of flexing generates heat ( = absorbs
    energy that has to be made up by the power source/engine/additional fuel).

    From a rolling resistance/fuel mileage standpoint, the best tire would
    be a rigid metal tire. You would essentially have an infinitly small
    contact point (line) and no flexing (compared to what you have with a
    rubber/jello tire). Wouldn't have very good traction, but if the *only*
    priority was fuel mileage, we would have metal tires.

    Bill Putney
    (To reply by e-mail, replace the last letter of the alphabet in my
    address with the letter 'x')
    Bill Putney, Jan 28, 2006
  4. tomkanpa

    MoPar Man Guest


    Subject: Re: Tire Pressure
    Date: Mon, 26 Jul 2004 21:00:55 -0400
    From: MoPar Man <>
    Newsgroups: alt.autos.dodge, rec.autos.makers.chrysler

    If you're a tire, you don't really care what car you're on. You care
    if you're mounted on the correct-sized rim, and then you care about
    how many pounds you're being asked to carry.

    It's actually a pretty dumb thing for car makers to put the tire PSI
    on the door jamb. Ok, well, they do know what tires they put on the
    car from the factory, so that's really the only *correct* situation
    where the door-jamb PSI spec is valid. Other than that, once you
    start putting on different tires (and different sizes) then the tire
    makers should have specs as to what a given tire should be pressurized
    to for a given weight to obtain the correct rolling profile.

    You need enough pressure so that the tire doesn't deform a lot
    (flatten-out) as it turns. That's a function of the weight of the car
    (and all cars are different). Too much air results in too little
    contact patch surface (and a hard ride, and too much center wear, but
    probably great fuel economy).

    Most passenger car tires know they're going to be carrying (3700 / 4 =
    ) 925 lbs, so you'd think that instead of the ridiculous "max
    pressure" rating on a tire that there'd be *the correct freeking PSI
    rating* for 1000 lbs load.

    What ever happened between Ford and Firestone's SUV tires? Was it
    proven that Ford's door-jamb rating was not correct for the particular
    tires that were blowing out on the highway?


    Subject: Re: Tire Pressure
    Date: Thu, 29 Jul 2004 23:03:02 -0400
    From: MoPar Man <>
    Newsgroups: alt.autos.dodge, rec.autos.makers.chrysler
    I've never seen a sidewall that has the "recommended" pressure
    stamped on it. They usually have the "max pressure".

    Tire makers seem reluctant to stamp a recommended pressure on tires.
    That seems odd. I'm sure they have a chart for every tire showing PSI
    on the Y axis and load (weight) on the X axis.

    The max load rating of a tire is proportional to the volume of air
    contained within the tire. The wider and taller a tire is, the higher
    the MAX PSI is (and the higher the MAX load is).


    RV forums sometimes have tire PSI discussions - such as in this link:


    The correct strategy here is to "(1) weigh the rig and (2) consult the
    tire manufacturers load/ pressure charts to find the right (air
    pressure) value."

    The PSI spec found on the door jam sticker of passenger cars assume
    that all tires have the same load/pressure chart (you tell me if
    that's a realistic assumption). It would be interesting to look at
    the tire inflation specs from 3 different 300M cars (one with 16"
    tires from the PHP package, one with the standard 17" wheels, and the
    third with 18" wheels from the 300M Special package). Are the PSI
    values different?

    Clearly, the PSI value on the door sticker has some over-all car
    weight in mind. If it's the GVWR (max gross weight which is a fully
    loaded car) then if you drive around with only yourself (and not 4
    other adult passengers) then inflating your tires to the spec value is
    technically over-inflation for what you're doing (it's safe, and it's
    probably going to give you good milage, but it's going to give you a
    harsh ride - and depending on the conditions of your roads your tire
    will experience more internal "injury" with higher PSI's).
    The goodyear link (above) has a chart which also indicates that up to
    5 extra PSI (for car tires) and 10 PSI (truck tires) are recommended
    for speeds between 65 and 75 MPH. The extra pressure at high speeds
    is desired to reduce the amount of flexing within the tire as it
    turns. There is energy (heat) generated within the tires as they
    turn, and the tires can only dissapate so much heat per unit time. At
    high speeds, you want the tire to generate less internal heat per
    revolution - so you give it more internal air pressure.


    Subject: Re: tire pressure?
    Date: Sun, 09 Oct 2005 11:14:15 -0400
    From: MoPar Man <>
    Newsgroups: rec.autos.makers.chrysler

    For some reason, automakers feel compelled to post tire PSI numbers on
    the car (door jab) or in the operating booklet. Maybe it's the law
    (however quaint).

    Technically the only time these numbers are right are when they
    pertain to the oem tire.

    The PSI you should put into any given tire will depend on the size of
    the tire (the VOLUME of space inside the tire), the load you will put
    on the tire, the width of the tire (which plays a role in the first
    item), the construction of the tire (thickness of sidewall), etc.

    Tire makers are notorious for not providing PSI vs load charts for
    their tires. They only indicate the max load and max pressure.

    No doubt there are liability fears over making the correct information
    more available or correctly posted/printed in appropriate locations.
    Give as little information as possible, and make it as vague and
    un-specific as possible. In the case of tire makers - if you don't
    have to provide it, then don't provide it.

    Vehicle makers would like to see you ride with tires that are more
    under-inflated than over-inflated. Under-inflated tires will absorb
    more road bumps and reduce the work of the suspension system to handle
    pot-holes and such. Automakers can essentially let under-inflated
    tires "eat" road problems and don't have to put extra cost and
    engineering into the suspension system to do this (more "protection"
    for aluminum alloy wheels for example if the tires absorb punnishing
    city potholes).

    Under-inflated tires also make it harder to roll larger vehicles
    (SUV's) so again they'd rather see an under inflation rather than over
    inflated. They may even purposely post low PSI numnbers on the door
    jamb decal to insure this (Ford vs Firestone for example).

    If the tires fail or wear prematurely, the owners will blame the tire
    (brand and maker) so the auto maker has nothing to lose by posting low
    numbers (ie it doesn't reflect poorly on them or have negative
    consequences unless you really dig into the issue, which few do).

    Ideally, you want a PSI that prevents (as much as possible) the tire
    flattening out as it rolls. Shape change for the tire equals internal
    heating and wear - which you want to avoid. Then again, you can't
    inflate tires to 100 psi either. Unless the roads you drive on are
    really bad, you're better off with 35 PSI vs 30 PSI for a typical
    passenger car tire (215/60/16 or 225/55/17). If you consistently car
    pool or load your vehicle with more than just the driver, then you are
    definately better off with 35 psi.

    For summer highway driving, the fact that the PSI goes up because the
    air inside gets warm is a good thing because

    1) highways are generally smoother than city/urban roads, so there are
    likely to be fewer dammaging or irritating bumps or other
    irregularites (over-inflated tires _might_ result in reduced road
    noise vs under inflated tires).

    2) fewer stops and starts on a highway also puts reduced load on the
    tires, very little lateral or shear forces because you're usually
    going in a straight line (curves are banked, etc).

    3) higher speeds means more rotations per second or per minute for the
    tires. Each rotation equals tire distortion or flattening, which
    equals internal heat generation and wear. Increase the PSI and you
    reduce the extent of tire deformation pre rotation, which reduces
    internal wear and heat generation (not to mention reduced rolling
    friction, and better fuel economy). When you get off the highway,
    internal air temperature will go down, so will PSI, and you shift the
    operating point of the tire to city/urban roads, where road handling
    (road contact, bouncing, cornering, etc) become more important.
    MoPar Man, Jan 29, 2006
  5. tomkanpa

    Ken Weitzel Guest

    Hi Bill...

    Perhaps not solid metal; how about solid metal with a
    non-inflated rubber surface instead?

    Solid metal hitting even a smallish stone on the road would
    mean that the stone would have to be either crushed or
    climbed over... which would cost energy :)

    Take care.

    Ken Weitzel, Jan 29, 2006
  6. tomkanpa

    Bill Putney Guest

    Heh heh! Maybe, but I think that solid metal would still be more
    efficient. Of course it is a moot point as neither would be used in the
    real world. Trains of course are what we're talking about - but they of
    course don't have the rocky surface to ride on like a road. Hmmm -
    that's the answer - steel wheels and steel roads! :)

    Bill Putney
    (To reply by e-mail, replace the last letter of the alphabet in my
    address with the letter 'x')
    Bill Putney, Jan 29, 2006
  7. tomkanpa

    Joe Guest

    Ignore the tires. You need to look in your owners manual at the inflation
    instructions. That'll answer your question.
    Joe, Jan 29, 2006
  8. tomkanpa

    Joe Guest

    I can't believe all the windy answers to this question. Nobody seems to
    notice that you read that off the tire. The manufacturer of the tire does
    not know which tire is on the front and which is on the back. For pete's
    sake. The directions from Chrysler will be in or on the vehicle. Read that.
    It may well be higher in front.
    Joe, Jan 29, 2006
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