Max Tire Pressure?

Awl--

I noticed that the recommended tire pressure for a vehicle is usually well
below the max. printed on the tire.
For example, on my new Honda Fit, the door says 32 psi, but iirc, the tire
says 40 or 45 psi.

What goes into this recommendation by the car mfr?
(Recall the Firestone tire debacle, which I heard was largely caused by Ford
recommending unrealistically low pressures, for a cushier ride.)

It seems to me that filling the tire to the *tire mfr's* printed max should
be OK, giving me 1. better mpg's 2. perhaps a bumpier ride, and 3.
perhaps less traction/longer braking distance than the car mfr's
recommended psi.

Since I drive pretty much slow as **** anyway (or with the "flow of
traffic"), mpg's are my main priority, as long as I'm not positively risking
my life.
I've noticed no uneven wear on previous cars, by filling to the max.
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all d'numbuhs

"Proctologically Violated©®" >
wrote in message ...
> Awl--
>
> I noticed that the recommended tire pressure for a vehicle is
> usually well below the max. printed on the tire.
> For example, on my new Honda Fit, the door says 32 psi, but iirc,
> the tire says 40 or 45 psi.
>
> What goes into this recommendation by the car mfr?
> (Recall the Firestone tire debacle, which I heard was largely caused
> by Ford recommending unrealistically low pressures, for a cushier
> ride.)

Wrong on two points. The recommended pressures were not
"unrealistically low" and they were not recommended "for a cushier
ride." When testifying before Congress, even the Firestone Executive
who was trying to shift the blame to Ford, admitted that the
recommended pressures were well within the industry standards for
tires of that size and type. The recommended pressure was similar to
the pressure recommended by Toyota and Nissan for SUVs with similar
size tires and loadings. Around half of 1996 Explorers were sold with
Goodyear tires instead of Firestone tires. The Goodyear tires got
exactly the same pressure recommendation and did not experience
abnormal failure rates. Ford also installed the same bad Firestone
tires on Ranger pick-ups, but with a higher pressure recommendation
for the rear tires, and they still failed at an excessive rate (my
Father had three of four fail on his Ranger). Ford picked the tire
pressures to reduce steering response and increase understeer. This
was to prevent people from breaking the rear end loose and possibly
spinning the vehicle or turning it over. Firestone was well aware of
Fords recommendation and made no objections until after the tires
started failing at an excessive rate. Firestone built defective tires
and tried to blame Ford. Ford isn't clean since they were stupid
enough to buy second rate tires from Firestone.

> It seems to me that filling the tire to the *tire mfr's* printed max
> should be OK, giving me 1. better mpg's 2. perhaps a bumpier ride,
> and 3. perhaps less traction/longer braking distance than the car
> mfr's recommended psi.

1) While increase tire pressure may reduce rolling resistance and
improve gas mileage, it is not a straight line effect. Past some
point, increasing the pressure further will not significantly affect
fuel economy. Given the pressure of CAFE regulations in the US in is
very unlikely that recommended tire pressures are so low as to greatly
reduce fuel economy.

2) Higher pressure will definitely result in a stiffer ride. You may
prefer this. On the other hand, higher pressures also make the tires
more vulnerable to damage from stones and broken pavement. Higher tire
pressure also result in a higher level of road shocks being
transmitted to the suspension on body.

3) Handling can be greatly affected by tire pressure - particularly
the front to rear differential. Excessively high pressure can
definitely increase braking distances and reduce cornering ability.
High pressures can also make the steering very "nervous."


> Since I drive pretty much slow as **** anyway (or with the "flow of
> traffic"), mpg's are my main priority, as long as I'm not positively
> risking my life.
> I've noticed no uneven wear on previous cars, by filling to the max.

Radial tires are much more forgiving of incorrect pressure than bias
ply tires. I own a Nissan Frontier, and even with the recommended
pressure in the rear tires I am experiencing excessive wear in the
middle of the tread. Nissan only recommend "one" pressure for the
tires. I am sure this is a "safe" pressure and one that results in
maximum gas mileage. However, for a lightly loaded truck, the pressure
is likely too high. Years ago, manufacturer's usually recommended
different pressures for different conditions (speed, load). In recent
years, this has been replaced by a one pressure fits all situations
recommendation. It is my belief that this pressure is probably higher
than necessary for most conditions since they have to allow for
maximum load and speed and the manufacturer's need to achieve the
highest possible mileage ratings for CAFE.

From http://www.michelinman.com/care/tip1.html :

"Recommended Pressure

"Always inflate your tires to the recommended pressure listed by your
vehicle's manufacturer. This information can be found in the owner's
manual and often on a placard located in the vehicle's door jamb,
inside the fuel hatch, or on the glove compartment door."

From http://www.goodyeartires.com/faqs/Inflation.html :

"How much air should I put in my tires?

"Proper inflation is the single most important part of tire care. The
inflation pressure on the side of the tire is the MAXIMUM operating
pressure. It is not necessarily the right inflation for your vehicle.
Always use the inflation recommended by the vehicle manufacturer. You
can find it in your owner's manual, posted on the edge of the driver's
door, on a door post or on the inside of the glovebox door. Always
check inflation when tires are COLD: when the vehicle has been driven
less than a mile or one hour or more after driving. Use a good quality
tire gauge. Note: It's natural for radial tires to have a slight bulge
in the sidewall at their proper inflation pressure. Check or adjust
inflation every few weeks, before any long trip or if traveling with a
heavy load. And don't forget to check the spare. Your Goodyear
retailer can answer any questions you may have about tire inflation."

From http://www.tiresafety.com/maint/maint_ipressure.asp (this is a
site linked from Firestone's home page):

"Inflation Pressure

"Proper inflation pressure is essential for achieving maximum
performance and mileage. Improper tire inflation pressure can cause
severe internal tire damage, which can lead to sudden tire failure and
resulting in serious personal injury or death.Improper inflation
pressure may result in rapid or irregular wear. Pressures should
always be checked when the tires are cold and at least monthly. Under
normal tire operation, approximately 1psi of tire pressure will escape
every month. Also, for every 10 degrees F change in ambient
temperature, tire pressure will change by approximately 1psi.

"Vehicle manufacturers list recommended tire pressures for original
vehicle tires in the owner's manual or on a placard on the end of the
driver's side door or in the glove box.

"For continuous high speed driving, tire pressures should be increased
by 3 to 5psi above the normal cold inflation recommended. However, for
passenger tires, never exceed the maximum inflation pressure molded on
the sidewall. The inflation pressure for light truck tires may exceed
that molded on the tire by 10psi. Any recommended front to rear
pressure differential should be maintained."

From
http://www.conti-online.com/generato...ntal/automobil...

"Tires are designed and built to provide many miles of excellent
service but must be maintained properly. The key element of proper
tire maintenance is maintaining the recommended tire inflation
pressure. The proper tire inflation pressure is recommended by the
vehicle manufacturer and can be found on the vehicle's tire placard or
in the vehicle owner's manual.

"Continental Tire recommends that the consumer check his/her tire
inflation pressure at regular intervals of at least once per month and
before every long trip or twice per month depending on local
regulations, customs, or conditions."

From http://www.coopertires.com/Flash/index.aspx :

"It's important to have the proper air pressure in your tires, as
underinflation can lead to tire failure. The "right amount" of air for
your tires is specified by the vehicle manufacturer and is shown on
the vehicle door edge, door post, glove box door, or fuel door. It is
also listed in the owner's manual."

Ed

C. E. White wrote:
> is likely too high. Years ago, manufacturer's usually recommended
> different pressures for different conditions (speed, load). In recent
> years, this has been replaced by a one pressure fits all situations
> recommendation. It is my belief that this pressure is probably higher
> than necessary for most conditions since they have to allow for
> maximum load and speed and the manufacturer's need to achieve the
> highest possible mileage ratings for CAFE.
>
> From http://www.michelinman.com/care/tip1.html :
>
<snip>
> "For continuous high speed driving, tire pressures should be increased
> by 3 to 5psi above the normal cold inflation recommended. However, for
> passenger tires, never exceed the maximum inflation pressure molded on
> the sidewall. The inflation pressure for light truck tires may exceed
> that molded on the tire by 10psi. Any recommended front to rear
> pressure differential should be maintained."

fwiw, my 2001 Trans Am's sticker has a 30psi rating for normal driving,
and a 38psi rating for sustained speeds above 100mph. I love that
sticker.

Ray

Proctologically Violated©® wrote:
> I noticed that the recommended tire pressure for a vehicle is usually well
> below the max. printed on the tire.
> For example, on my new Honda Fit, the door says 32 psi, but iirc, the tire
> says 40 or 45 psi.

I like that the recommended pressure isn't higher than the tires' capacity.
But that's just me.

--
We can't possibly imprison 300 million Americans for not paying their taxes,
so let's grant all of them amnesty NOW!

"Ray" > wrote in message
...
> C. E. White wrote:
>> is likely too high. Years ago, manufacturer's usually recommended
>> different pressures for different conditions (speed, load). In
>> recent years, this has been replaced by a one pressure fits all
>> situations recommendation. It is my belief that this pressure is
>> probably higher than necessary for most conditions since they have
>> to allow for maximum load and speed and the manufacturer's need to
>> achieve the highest possible mileage ratings for CAFE.
>>
>> From http://www.michelinman.com/care/tip1.html :
>>
> <snip>
>> "For continuous high speed driving, tire pressures should be
>> increased by 3 to 5psi above the normal cold inflation recommended.
>> However, for passenger tires, never exceed the maximum inflation
>> pressure molded on the sidewall. The inflation pressure for light
>> truck tires may exceed that molded on the tire by 10psi. Any
>> recommended front to rear pressure differential should be
>> maintained."
>
> fwiw, my 2001 Trans Am's sticker has a 30psi rating for normal
> driving, and a 38psi rating for sustained speeds above 100mph. I
> love that sticker.

Wow, I am surprised GM doesn't get sued over that one. I mean if the
sticker says you can go 100, you probably should....

Do you remember when manufacturers were installing tires with an 85
mph maximum speed rating? I thought that was insane, even though the
National Speed Limit was 55. Now things have gone in the opposite
direction - cars come with tires rated for 135 mph, even if they have
governors limiting them to 105. And tire stores force you to buy the
higher rated tires even if you never plan to go over 80....

Ed

> It seems to me that filling the tire to the *tire mfr's* printed max
> should be OK, giving me 1. better mpg's 2. perhaps a bumpier ride,
> and 3. perhaps less traction/longer braking distance than the car
> mfr's recommended psi.

Tires are fairly "generic" and are designed so that they can be used
on a host of different vehicles of different weights. The
manufacturer sets a recommended pressure for the specific vehicle
taking in to account things like vehicle weight and handling
characteristics.

If you run the tires up to the max limit you end up riding on the
middle of the tire. Now the tires aren't going to last as long as
they should and your handling / braking goes to pot. Any potential
savings are quickly negated by having to replace the tires early.

Vehicle manufacturers have nothing to gain by intentionally making
their vehicles get worse gas mileage. If you want to save gas I would
work on keeping the tires properly inflated, make sure the engine is
in good tune and most importantly work on driving habits that might be
costing you gas mileage.

Steve B.

Proctologically Violated©® wrote:
> Awl--
>
> I noticed that the recommended tire pressure for a vehicle is usually well
> below the max. printed on the tire.
> For example, on my new Honda Fit, the door says 32 psi, but iirc, the tire
> says 40 or 45 psi.
>
> What goes into this recommendation by the car mfr?
> (Recall the Firestone tire debacle, which I heard was largely caused by Ford
> recommending unrealistically low pressures, for a cushier ride.)
>
> It seems to me that filling the tire to the *tire mfr's* printed max should
> be OK, giving me 1. better mpg's 2. perhaps a bumpier ride, and 3.
> perhaps less traction/longer braking distance than the car mfr's
> recommended psi.
>
> Since I drive pretty much slow as **** anyway (or with the "flow of
> traffic"), mpg's are my main priority, as long as I'm not positively risking
> my life.
> I've noticed no uneven wear on previous cars, by filling to the max.


We run mostly oversized tires on our off road Jeeps so we need a way to
find the compromise for best traction and best handling vs gas mileage
and safety.

We use the chalk method of running a few lines across the tread and
driving straight, then stopping to see how much is left. The best for
grab is usually no chalk except about 3/8" at each edge. This is lower
than the mfg vehicle spec which gives about 1" on each edge.

When my 33x9.5" tires to use one specific case are at 26 psi I have all
7.5" of tread touching but a soft sidewall. Good for slow speed only.

At the vehicle's spec of about 30 psi, I have about 3/4" of chalk on
each side. At the max of 45 psi for the tire maker, I only have 3 1/2"-
4" of tread touching the ground! This leaves a gap that just screams
for a little water to get lift off!

I run at 28-29 psi and get good wear with decent handling and decent
enough mileage which is a compromise. If loaded I run at 32 psi though.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

Proctologically Violated©® wrote:
> Awl--
>
> I noticed that the recommended tire pressure for a vehicle is usually
> well below the max. printed on the tire.
> For example, on my new Honda Fit, the door says 32 psi, but iirc, the
> tire says 40 or 45 psi.
>
> What goes into this recommendation by the car mfr?
> (Recall the Firestone tire debacle, which I heard was largely caused
> by Ford recommending unrealistically low pressures, for a cushier
> ride.)
> It seems to me that filling the tire to the *tire mfr's* printed max
> should be OK, giving me 1. better mpg's 2. perhaps a bumpier ride,
> and 3. perhaps less traction/longer braking distance than the car
> mfr's recommended psi.
>
> Since I drive pretty much slow as **** anyway (or with the "flow of
> traffic"), mpg's are my main priority, as long as I'm not positively
> risking my life.
> I've noticed no uneven wear on previous cars, by filling to the max.

Uhhh, the "maximum" pressure is that at which the tire, if inflated beyond,
will be in danger of blowing, erupting, going flat, etc etc.. IOW the tires
become dangerous beyond that pressure. Using the max pressure is lunacy.
Over inflated tires have been known to explode loudly and with destructive
forces.

C. E. White wrote:
> "Ray" > wrote in message
> ...
>> C. E. White wrote:
>>> is likely too high. Years ago, manufacturer's usually recommended
>>> different pressures for different conditions (speed, load). In
>>> recent years, this has been replaced by a one pressure fits all
>>> situations recommendation. It is my belief that this pressure is
>>> probably higher than necessary for most conditions since they have
>>> to allow for maximum load and speed and the manufacturer's need to
>>> achieve the highest possible mileage ratings for CAFE.
>>>
>>> From http://www.michelinman.com/care/tip1.html :
>>>
>> <snip>
>>> "For continuous high speed driving, tire pressures should be
>>> increased by 3 to 5psi above the normal cold inflation recommended.
>>> However, for passenger tires, never exceed the maximum inflation
>>> pressure molded on the sidewall. The inflation pressure for light
>>> truck tires may exceed that molded on the tire by 10psi. Any
>>> recommended front to rear pressure differential should be
>>> maintained."
>>
>> fwiw, my 2001 Trans Am's sticker has a 30psi rating for normal
>> driving, and a 38psi rating for sustained speeds above 100mph. I
>> love that sticker.
>
> Wow, I am surprised GM doesn't get sued over that one. I mean if the
> sticker says you can go 100, you probably should....
>
> Do you remember when manufacturers were installing tires with an 85
> mph maximum speed rating? I thought that was insane, even though the
> National Speed Limit was 55. Now things have gone in the opposite
> direction - cars come with tires rated for 135 mph, even if they have
> governors limiting them to 105. And tire stores force you to buy the
> higher rated tires even if you never plan to go over 80....
>
> Ed

Some tires are now rated for 75 mph. You're thinking about specials or
European tires.

Steve B. wrote:
>> It seems to me that filling the tire to the *tire mfr's* printed max
>> should be OK, giving me 1. better mpg's 2. perhaps a bumpier ride,
>> and 3. perhaps less traction/longer braking distance than the car
>> mfr's recommended psi.
>
> Tires are fairly "generic" and are designed so that they can be used
> on a host of different vehicles of different weights. The
> manufacturer sets a recommended pressure for the specific vehicle
> taking in to account things like vehicle weight and handling
> characteristics.
>
> If you run the tires up to the max limit you end up riding on the
> middle of the tire. Now the tires aren't going to last as long as
> they should and your handling / braking goes to pot. Any potential
> savings are quickly negated by having to replace the tires early.

They are also more prone to punctures, rim problems, carcass destruction in
use, and will go out of balance extremely quickly when a band breaks. The
TIRE max is just that; a MAX! Riding on it is foolhardy.

>
> Vehicle manufacturers have nothing to gain by intentionally making
> their vehicles get worse gas mileage. If you want to save gas I would
> work on keeping the tires properly inflated, make sure the engine is
> in good tune and most importantly work on driving habits that might be
> costing you gas mileage.
>
> Steve B.