Tire Pressure

I apologize for the crosspost...

I have a pair of Neons, an '03 and an '04, and the tire pressure was getting
low in the '03. I've never checked it since we've had it, but it was reading
about 27 psi at each wheel. The door sticker reads 32 psi but the tire
sidewall reads a max of 44 psi. So I'm wondering what should the pressure be
set at? In years past, I always went about 4-6 psi less than the max listed
on the tire to account for temperature changes, altitude, etc. So I ended up
going with 38 psi all around. The ride is much firmer and I'm wondering if
it's meant to be at 32 psi after all.

Any suggestions?

Thanks,
NG

 

I could be wrong about this, but my view has always been that automotive
manufacturers are paid to engineer these things to a specification
exacting enough to provide the best longevity at the least cost with the
tools and technologies they are given (permitted?) to use. With this in
mind, they're not going to cheap out on tire research when that's a very
extremely important aspect of the overall vehicle. They're going to do
the research, do the stress testing, do the road testing, and they're
eventually going to get to the right tire pressure that permits for things
like road temperature, vehicle weights, etc., and therefore all you need
to do is fill the tires to the point they tell you to. MY far-from-expert
advice would be not to second-guess an engineer who probably earns a hell
of a lot more than any of us do, and to trust what they tell you is
appropriate for your vehicle.

OTOH, if you're an automotive professional with years of training and very
expensive education in the matter, you could probably customize every
little nuance of your ride to fit your driving style, but what they came
up with is pretty much guaranteed to work for all sane driving styles.

FWIW, I fill mine to 32 psi. Mine hold air with no problem. About every
6 months or so I have to put just a couple psi in, but not much. If
you haven't lost but 5 psi over a year and a half, that's not bad at all.
Just put it back at 32 and keep a regular check on it.

As for altitude, even at sea level we're only under 14 psi or so, so I'd
imagine at 50,000 feet it would only drop 7 psi. We can't even get above
10,000 without the need for oxygen, and not much higher before we need
pressurization, but I don't see altitude really affecting automotive tires
to the point you need to concern yourself with it.

Which also makes me wonder if you even need to worry about road
conditions: do a simple test... take the psi reading before you go
driving, and then take the family on a nice scenic drive or something
(good 30 minutes or so at highway speed should do the trick) and when you
get back home, check the pressure again while the tires are still warm. I
doubt you'd see more than 4 or 5 psi difference, although it's a guess.

Again, I'm no expert. HIH.

CJ

 

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?

 

Of course it's meant to be at 32. Why do you think they put that sticker
on the door?

Did you ever think to check the owner's manual for information?

 

Having been in the tire business for over thirty years and have seen
everything from the firestone 500 problem of the sixties to the present
problems. The air pressure issue is very important. You should check your
pressure based on the car makers recommendations. The pressure should be
checked once a month after the car has set for over 8 hours.
The proper way to do this as follows.
check the recommended air pressure from the door jam, for this example lets
use 32 psi.
Measure the air pressure with a reliable gauge.
Write down the pressure for each wheel position. i.e.; L/F 26 psi, R/F 29
psi etc.
Then if you do not have your own compressor drive to the nearest service
station that does and measure the pressure.
IE the L/F now reads 29 psi and increase of 3 psi
Inflate that tire to 35 psi ( 32 plus the 3 psi from the difference of being
cold.)
Then the following morning re-check

The problem with Ford and Firestone was 2 fold. Ford had recommended a low
air pressure of 25 psi which would have been no problem if the average
consumer checked his air pressure regularly. (not)
The failure mainly occurred in high temperature areas where tires heat up
faster.Also the fact that other tires had failed on the ford products leads
one to believe that it was a design flaw on Ford's part. If you look at the
location of the rear exhaust in relation to the rear tire it appears to be
very close thereby adding extra heat .

 

25 PSI?!?! That tire is rated for something like 40-44 PSI! It would seem
to me that this would result in *serious* overheating, even at Ford's
recommended tire pressure!

FWIW, I generally go by the tire's sidewall, and go to the max. Usually I
have a decent ride and decent gas mileage.

 

About 7 months ago I had new tires put on my wifes '00 Taurus at Firestone.

The tires started balding about 2 months ago, so I took it in.

They wouldn't warranty the tires because I inflated them to what it said on
the sidewall of the tire.
They told me to go by the vehicle manufacturers specifications no matter
what, or the warranty is void.

Now, I'm not sure if they were just scamming me or what, but I've since gone
by the factory manual and they haven't given me a problem since.

 

I've regarded the owner's manual recommendations as a starting point
only, ever since the 1960s when tire pressures of 24 psi or so were
recommended by ride-conscious automakers, much to the detriment of handling.

A point of warning: if you go by the door sticker alone, you may not be
adhering to manufacturer's recommendations in "high speed" driving,
which turns out to be what a lot of us actually do. Read your owner's
manual. Note the example which follows.

I offset front vs. rear tire pressures to assist in adjusting the
handling of my cars. My 1995 Dodge Intrepid departs from stock only in
the replacement of one rear sway bar bushing on each side with
polyurethane instead of the factory rubber parts; and revised tire
sizing, replacing the factory's P205/70R15 with P215/65R15. The door
sticker with tire pressures says 32 lbs., front and rear; HOWEVER the
owner's manual says for sustained speeds of 75mph or more (and these
days on expressways, 75mph is pretty much normal speed) the pressures
should be increased to 35 lbs., front and rear. I run 39 lbs. on the
front and 35 lbs. on the rear; this reduces understeer by increasing
front tire grip, and with the "quicker" action of the rear sway bar due
to the harder polyurethane bushing, it makes it easier, actually fun, to
"toss" this relatively large vehicle into corners.

Near my home is a right/left S-bend, the left being sharper than the
right, the whole having a mild dip at its entry. Box-stock, the car had
a response which was safe but felt slow and sluggish here, especially at
the transition in the middle of the S-bend where the front tires would
plow. With the higher front tire pressure and the replacement bushings,
response is quick and adhesion much better balanced between front and
rear. In the middle of the S-bend, a quick turn of the wheel to the
left is all it takes to bring the car around and exit straight out,
while opening throttle and accelerating out. What's more, the whole,
stable demeanor of the tires and chassis are such that I could do that
kind of thing all day. And the tires wear flat across the tread
grooves, too, even on the front.

 

Pretty bold assumptions there. Like everything, the recommended
pressure is a compromise of many things. The ideal pressure for gas
mileage will not be the ideal pressure for tread longevity will not be
the ideal pressure for handling, will not be the ideal pressure for ride
smoothness, etc., etc. A given manufacturer may put higher priority on
soft ride (claimed by some to be an over-compromise that created or
contributed to the more recent Firestone debacle), while the same
manufacturer may put higher priority on handling on a different vehicle
or even on a different version of the same vehicle (with, say, a
handling package option). *Everything* is a compromise. The final
answer depends on the weighting factors put on the various aspects of
vehicle/tire ownership. Nothing wrong with an owner putting a higher
priority on handling and tire longevity/even tread wear and cooler
running temperatures and putting 2 to 4 psi higher than the recommended
pressure. Going lower than that recommended rarely makes much sense.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

The sidewall pressure is the MAXIMUM allowabel pressure, which would
be recommmended if operating the tire at MAXIMUM load.
Any higher pressure and/or load and the tire is in danger of rupture.
The car manufacturer /engineers decide what the proper tire pressure
is for a given tire size when installed on the vehicle to give best
combination of ride, handling, and tire-wear. Deviating from that
pressure without a very good understanding of the loads and conditions
involved is foolishness.
If you understand the ramifications, small changes can safely be made.

 

The pressure on the door jam, of course.

I suggest you follow the car manufacturers recommendations. In other words
the sticker on the door jam.

 

This can lead to excessive wear in the center of the tire.

 

Yes, I think Ford really messed up if that was their recommended
pressure for a vehicle the size of the Explorer.

I tend toward the max also, but keep in mind that this is really
indicated if you have the tire loaded to the max load specified along
with the max pressure. A lower pressure for a lower load isn't a bad thing.


Matt

 

Did they wear only in the center of the tread?

That's a pretty pathetic excuse. However, there must be more too it
because even being overinflated by that amount wouldn't wear a tire to
the point of being bald in only 5 months ... unless you drive 10,000
miles per month.

Is that clause in their written warranty? If not, then they need to
honor their warranty.

The car maker's recommendations are typically a decent compromise, but
they are a compromise. If you run your vehicle heavily loaded all of
the time, then you likely need higher pressure. If you drive alone all
of the time, the the manufacturer's recommendations are probably fine.
I tend towards the max on the tire, especially on my pickup, and have
have good tire life with only a slightly harsher ride.


Matt

 

The sticker on the door gives a pressure which is a compromise of
ride/rolling resistance/handling/tread life. Generally, you won't go
wrong by using that pressure. Going higher than the 32 psi (but never
higher than the number on the sidewall) will improve all factors in the
compromise except ride quality. If you don't mind a harder ride, going
a little higher than 32 is not a bad thing. The pressure should always
be checked with the tires cold. If you don't check and fill them at
home, do it at a place close to home.

 

Hiya all,

The PT Cruiser states 38psi but to use 30 if I want a more comfortable ride.
I put 40 in when towing.
The Jag says 28psi for regular driving and 35 for sustained speeds over
100mph.
The '84 Granada says 28psi but I like to drive it sideways and I wear the
edges off so I use 32psi

It all depends on what the tyre size is, the weight of the car and your
driving style. The maximum pressure on the tyre itself is a guide for the
tyre and not the car and should not be exceeded or it might go bang. If you
want more pressure get a different tyre.

Watch the wear pattern and this will give you a clue as to the pressure you
need.

hope this helps

pottsy UK

 

I agree.

On my LeBaron, I keep them inflated to what the sidewall says to inflate
them to.

The reason being is that the Chrysler shop manual (Which wasn't cheap) says
that any speed under 65mph to inflate them to 35 psi. Anything over 65mph,
to go by the tire.

Soooooo, if I have to take it back to Firestone, I'll show them this in the
shop manual, and wait for their next piece of shit excuse...

 

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).

http://www.goodyear.com/truck/pdf/databook/5113_Lsec_V.pdf
http://www.tirerack.com/tires/tiretech/tirespecskey.jsp#maxload
http://www.dodge.com/bodybuilder/2004/docs/intro/tirecharts.pdf

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

http://www.rv.net/forums/index.cfm/fuseaction/thread/CFB/1/TID/246059.cfm

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.

 

The sidewall doesn't say what to inflate them to. The sidewall says the
*maximum allowable pressure* for that specific tire.

Most you could've spent on it is $90, and that's if you didn't shop
around. Any idea what a *factory* service manual costs for an '85 Volvo?
The answer has four digits *before* the decimal point.

Every tire they make is a piece of shit excuse. Next time get
BFGoodriches.

DS

 

But why would you WANT an 85 Volvo?

Matt