I've been going by the rating on the side of the tire for, oh, 30 years or
so...

isn't the sill label for ride comfort only.
I too go by what's on the tire, max 35 psi.

We've talked here before about max cold tire inflation pressure.
Here's a fascinating article from Officer.com .  Some police
officers drive with maximum air pressure in their tires as shown
on the tire sidewall, not the door sill or the owner's manual.

Here's a link to the article for some great photographs:

http://www.officer.com/article/article.jsp?siteSection=19&id=27281

and here's the text in case the page is deleted in the future:

Driving Under Pressure

Proper Tire Pressure Could Save Your Life

Posted: Wednesday, December 21, 2005

SGT. DAVE STORTON
EVOC Contributor

Officer.com

How many officers check the tire pressure on their patrol car
on a regular basis? We all seem to be great at checking that
the lights and siren work, because the time to find out they
don't work is not when you get a Code 3 call. Likewise, the
time to find out your tire pressure is too low is not when you
are in a pursuit and trying to take a corner at high speed.

What is proper pressure?

The proper tire pressure for the Police Crown Victoria is 44 psi.
If you look on the sidewall of the tire, you will see that it
lists 44 psi max pressure.  Regardless of what vehicle you have,
use the maximum pressure listed on the sidewall. Higher pressure
results in better performance, decreased tire wear, and it
lessens your chance of hydroplaning at a given speed. This number
on the sidewall lists the maximum amount of pressure you should
ever put in the tire under normal driving conditions.  Pursuits
and Code 3 responses are not normal driving conditions. Many
agencies maintain tire pressure at 35 psi since this is what
is listed in the owner's manual and on the door placard. The
reason the owner's manual lists 35 psi is because we get the
same manual as the civilian version of the Crown Victoria. The
police version, however, is fully loaded with communications
equipment, a cage, and your gear. You are not looking for a soft
and cushy ride, you want performance.

Myths about pressure

Let^Ã?s put to rest some common misconceptions. The tires will
not balloon out creating a peak in the center portion of the
tread when tire pressure is above 35 psi. There is a steel belt
that prevents this from happening. Also, you are not
overstressing the tire with higher pressure, and the tire will
not be forced off the rim with higher pressure. The picture
above is Bobby Ore of Bobby Ore Motorsports driving a Ford Ranger
on two wheels. The tires on the left side have 100 psi in them,
and they happen to be tires and rims from a 1999 Crown Victoria!
This is a dramatic example of how pressure holds the tire in
shape, and how much stress a tire can handle.

Performance

If you were able to watch a tire as it travels across the
ground at high speed, you would see that it deflects to one
side during cornering. The faster you are going through a corner,
the more tire deflection you get. As the tire deflects over onto
the sidewall, you get less traction and more of a tendency to
understeer or oversteer. This could spell disaster when
negotiating a corner at high speed during a pursuit or a Code 3
run. Higher pressure keeps the tire from deflecting onto the
sidewall as much, which keeps more of the treaded portion on
the road.

A good demonstration for EVOC instructors is to have students
drive a high-speed course in a vehicle with 32 to 35 psi. Then
have them run the same course with 44 to 50 psi in the tires.
The student will experience a marked difference in performance.
Having officers experience this difference in vehicle
performance is much more effective than just telling them to
check their tire pressure.

Hydroplaning

When a tire rolls across a road covered with water, the tire
tread channels water away so the rubber remains in contact with
the road. The factors that affect hydroplaning are speed, and
water depth. Conventional wisdom says that vehicles will hydroplane
in as little as 1/16th of an inch of water. Not so coincidentally,
legal tread depth is 1/16th of an inch.

Tire manufactures and the Association of Law Enforcement Emergency
Response Trainers International (ALERT) have shown that tires have
more of a tendency to hydroplane when pressure is low. This
happens because the tire footprint (the portion of the tire
actually in contact with the road) is larger. For those of you
who water ski, think of which is easier to get up on: a fat ski
or a skinny ski. More tire surface in contact with the water
makes it easier to hydroplane, just as it is easier to water ski
on a fat ski. Also, a soft tire can be pushed in more by the
pressure of the water on the center portion of the tread. This
results in less rubber in contact with the road.

Tire wear

Much better tire wear results from maintaining proper pressure.
Tires with lower pressure will wear off the outside of the tread
faster from the deflection of the tire during cornering, and the
tires will heat up more from increased road friction. This is one
of the factors that caused the failure of a certain brand of
tires on Ford Explorers some years ago. In 1999 the San Jose
Police Department realized a significant cost savings by
increasing the pressure in the training fleet to 50 psi. They
soon followed up by increasing the pressure in the patrol fleet
to 44 psi. For liability reasons, most agencies are reluctant to
exceed the maximum pressure listed on the tire for actual patrol
vehicles, but they reap the cost saving when going to 50 psi on
training vehicles.

Next time you inspect your vehicle, make sure you check your
tire pressure since your ability to performance drive is
significantly affected by it. You are not driving to the store
to get a loaf of bread! You may be called upon to chase a
dangerous criminal or respond to assist another officer in
trouble. You don't wonder whether or not your gun is loaded
before you hit the street; don't wonder whether your tire
pressure is correct once the pursuit starts. Check your tires
routinely, just as you do with all other critical equipment.
--
Sgt. Dave Storton is the Director of the San Jose Police
Academy, and he holds a Master's Degree in Adult Education.
He is the lead instructor for the Emergency Vehicle Operations
Course (EVOC) at the San Jose Police Academy, and is a lead
instructor for the local regional academy. He teaches EVOC
instructor courses, advanced EVOC instructor courses, off road
EVOC, counter-terrorist / dignitary protection driving, and
motion picture stunt driving. Dave has trained over 3,500
drivers.

Photo courtesy of Bobby Ore Motorsports

Most people don't know they have to let their tires cool before they fill
them... and most oil companies who fill tires dont take the time to worry
about that either.

We've talked here before about max cold tire inflation pressure.
Here's a fascinating article from Officer.com .  Some police
officers drive with maximum air pressure in their tires as shown
on the tire sidewall, not the door sill or the owner's manual.

Here's a link to the article for some great photographs:

http://www.officer.com/article/article.jsp?siteSection=19&id=27281

and here's the text in case the page is deleted in the future:

Driving Under Pressure

Proper Tire Pressure Could Save Your Life

Posted: Wednesday, December 21, 2005

SGT. DAVE STORTON
EVOC Contributor

Officer.com

How many officers check the tire pressure on their patrol car
on a regular basis? We all seem to be great at checking that
the lights and siren work, because the time to find out they
don't work is not when you get a Code 3 call. Likewise, the
time to find out your tire pressure is too low is not when you
are in a pursuit and trying to take a corner at high speed.

What is proper pressure?

The proper tire pressure for the Police Crown Victoria is 44 psi.
If you look on the sidewall of the tire, you will see that it
lists 44 psi max pressure.  Regardless of what vehicle you have,
use the maximum pressure listed on the sidewall. Higher pressure
results in better performance, decreased tire wear, and it
lessens your chance of hydroplaning at a given speed. This number
on the sidewall lists the maximum amount of pressure you should
ever put in the tire under normal driving conditions.  Pursuits
and Code 3 responses are not normal driving conditions. Many
agencies maintain tire pressure at 35 psi since this is what
is listed in the owner's manual and on the door placard. The
reason the owner's manual lists 35 psi is because we get the
same manual as the civilian version of the Crown Victoria. The
police version, however, is fully loaded with communications
equipment, a cage, and your gear. You are not looking for a soft
and cushy ride, you want performance.

Myths about pressure

Let^Ã?s put to rest some common misconceptions. The tires will
not balloon out creating a peak in the center portion of the
tread when tire pressure is above 35 psi. There is a steel belt
that prevents this from happening. Also, you are not
overstressing the tire with higher pressure, and the tire will
not be forced off the rim with higher pressure. The picture
above is Bobby Ore of Bobby Ore Motorsports driving a Ford Ranger
on two wheels. The tires on the left side have 100 psi in them,
and they happen to be tires and rims from a 1999 Crown Victoria!
This is a dramatic example of how pressure holds the tire in
shape, and how much stress a tire can handle.

Performance

If you were able to watch a tire as it travels across the
ground at high speed, you would see that it deflects to one
side during cornering. The faster you are going through a corner,
the more tire deflection you get. As the tire deflects over onto
the sidewall, you get less traction and more of a tendency to
understeer or oversteer. This could spell disaster when
negotiating a corner at high speed during a pursuit or a Code 3
run. Higher pressure keeps the tire from deflecting onto the
sidewall as much, which keeps more of the treaded portion on
the road.

A good demonstration for EVOC instructors is to have students
drive a high-speed course in a vehicle with 32 to 35 psi. Then
have them run the same course with 44 to 50 psi in the tires.
The student will experience a marked difference in performance.
Having officers experience this difference in vehicle
performance is much more effective than just telling them to
check their tire pressure.

Hydroplaning

When a tire rolls across a road covered with water, the tire
tread channels water away so the rubber remains in contact with
the road. The factors that affect hydroplaning are speed, and
water depth. Conventional wisdom says that vehicles will hydroplane
in as little as 1/16th of an inch of water. Not so coincidentally,
legal tread depth is 1/16th of an inch.

Tire manufactures and the Association of Law Enforcement Emergency
Response Trainers International (ALERT) have shown that tires have
more of a tendency to hydroplane when pressure is low. This
happens because the tire footprint (the portion of the tire
actually in contact with the road) is larger. For those of you
who water ski, think of which is easier to get up on: a fat ski
or a skinny ski. More tire surface in contact with the water
makes it easier to hydroplane, just as it is easier to water ski
on a fat ski. Also, a soft tire can be pushed in more by the
pressure of the water on the center portion of the tread. This
results in less rubber in contact with the road.

Tire wear

Much better tire wear results from maintaining proper pressure.
Tires with lower pressure will wear off the outside of the tread
faster from the deflection of the tire during cornering, and the
tires will heat up more from increased road friction. This is one
of the factors that caused the failure of a certain brand of
tires on Ford Explorers some years ago. In 1999 the San Jose
Police Department realized a significant cost savings by
increasing the pressure in the training fleet to 50 psi. They
soon followed up by increasing the pressure in the patrol fleet
to 44 psi. For liability reasons, most agencies are reluctant to
exceed the maximum pressure listed on the tire for actual patrol
vehicles, but they reap the cost saving when going to 50 psi on
training vehicles.

Next time you inspect your vehicle, make sure you check your
tire pressure since your ability to performance drive is
significantly affected by it. You are not driving to the store
to get a loaf of bread! You may be called upon to chase a
dangerous criminal or respond to assist another officer in
trouble. You don't wonder whether or not your gun is loaded
before you hit the street; don't wonder whether your tire
pressure is correct once the pursuit starts. Check your tires
routinely, just as you do with all other critical equipment.
--
Sgt. Dave Storton is the Director of the San Jose Police
Academy, and he holds a Master's Degree in Adult Education.
He is the lead instructor for the Emergency Vehicle Operations
Course (EVOC) at the San Jose Police Academy, and is a lead
instructor for the local regional academy. He teaches EVOC
instructor courses, advanced EVOC instructor courses, off road
EVOC, counter-terrorist / dignitary protection driving, and
motion picture stunt driving. Dave has trained over 3,500
drivers.

Photo courtesy of Bobby Ore Motorsports

There are two figures on the sidewall of every tire. " MAX pressure 44
psi at MAX load 1300 lbs."  The 44 psi is for a tire loaded to it's
maximum load.  Tire profile is the reason why the pressure is adjusted
according to the weight on the tire.

Then why do some tires wear more in the center than on the edges?  Why
do some tires wear more on the edges than the center?

The steel belt doesn't keep the tread flat on the road, it keeps the
tread from squirming or twisting around side to side.  That is the
primary reason why radial tires get much better tread wear than
bias-ply tires.  It's also why radial tires run cooler than bias-ply.

Sorry but tire construction has more of an effect on the tire rolling
over than pressure.  If the tire is severely under inflated (<25 psi)
then the above statement would be true but the number of plys in the
sidewall determine how much the tire will deflect in cornering more
than being under inflated by 10 lbs will effect it.

Check with NASCAR pit crews and you'll find that they run their tires
at pressures that would ruin your street tires (10-15 psi) and they
don't have problems with the tire rolling over and lifting the tread
off the road.  True, their tires are not street tires but they use the
same construction as street tires.

Correct

Again he is correct.

Yes, but the proper pressure isn't the pressure listed on the sidewall
of the tire unless you also are loading it to it's maximum load.

This is just wrong.  Yes, they realized a cost savings in their
training fleet but those cars are NOT used on the roads to train the
officers.  They are used on closed courses to teach defensive driving
and avoidance maneuvers.  They also are used to teach other driving
techniques that are only rarely used on public roads.  The cars are
not driven at speed for 4 or 5 hours a day.

-------------------------

I drove fleet trucks (3/4 ton, 4 wheel drive pickups) for a number of
years.  I went through at least 10 or 12 trucks putting around 100,000
to 120,000 miles on each one.  My truck tires would last from 80,000
to 120,000 miles but I did two things.  I weighed each axle when fully
loaded and then I put the proper amount of air in the tire for the
load it was carrying.  The sidewall might have listed 80 psi but my
pressures were more like 65 to 70 psi.  The only times I had tires
fail were due to punctures or sidewall cuts (we did a lot of travel on
railroad right-of-way).  I maintain my personal car tires the same
way, I adjust the pressure according to the load I'm carrying.

Now, before some smart-a.s points it out.  No, I don't adjust tire
pressure as I use gas or when we buy groceries.  But if we're going on
a trip with a lot of luggage in the trunk, yes I will add from 3 to 5
psi to the rear tires.

A word of advise.  If you use 10 minute oil changes places for oil
changes, don't let them mess with your tire pressure.  I've had these
folks let air out of my truck tires so that I only had 35 psi in them
instead of the 65 or 70 they needed.

Jack

I have been misled by so many cartoons.  Or does that only apply to
large white tires on 8" rims?