Tire pressure

[BobN]

Last Friday I put a set of Goodyear Comfort Treads on my 98 OBLW. The tire
dealer filled them to 29 psi as spec'd on the door sticker, but the tires
looked really flat at that pressure. The maximum recommended pressure on
the sidewall is 44 psi but the dealer said that would completely wrong for
the car. That got me to thinking.

I reckon that the tire is designed to hold a certain shape under load, and
depending on the weight of the car that would involve different pressures
for different cars, and the maximum on the sidewall would refer to the
pressure at the highest rated load. Different tires also have different
sidewall stiffnesses, so it doesn't make sense to go by the OEM door sticker
when changing to a different brand/model tire. If I put too much pressure
in the tire it will have the wrong shape and not handle correctly. But I
like to have the highest reasonable pressure to decrease rolling resistance,
since most of my miles are highway.

Is there a way to calculate the correct pressure for a specific tire and car
weight (assuming typical passenger/cargo load and driving conditions)?

 

[Grolsch]

I've got the Goodyear Triple Treads on my 04 Forester and I run them at ~
34-36psi. This is a good compromise between fuel economy and comfort. The
lower oem suggested pressure would give you greater comfort (ie: not as
jittery) over bumps etc, but sacrifices handling and fuel economy. Greater
than 40psi would probably be too coarse of a ride for most drivers. I assume
that quiet and comfort are important to you based on your tire choice. Try
34psi and adjust from there. You'll know when it's too much when the wheels
feel like solid rubber over railroad tracks etc.

 

[John Rethorst]

Last Friday I put a set of Goodyear Comfort Treads on my 98 OBLW. The tire
dealer filled them to 29 psi as spec'd on the door sticker, but the tires
looked really flat at that pressure. The maximum recommended pressure on
the sidewall is 44 psi but the dealer said that would completely wrong for
the car. That got me to thinking.

I reckon that the tire is designed to hold a certain shape under load, and
depending on the weight of the car that would involve different pressures
for different cars, and the maximum on the sidewall would refer to the
pressure at the highest rated load. Different tires also have different
sidewall stiffnesses, so it doesn't make sense to go by the OEM door sticker
when changing to a different brand/model tire. If I put too much pressure
in the tire it will have the wrong shape and not handle correctly. But I
like to have the highest reasonable pressure to decrease rolling resistance,
since most of my miles are highway.

Is there a way to calculate the correct pressure for a specific tire and car
weight (assuming typical passenger/cargo load and driving conditions)?

I wonder if Goodyear has recommendations other than the max pressure?

 

[Edward Hayes]

Generally the manufactures recommended tire pressure is the all around
best for safety, comfort and life. I do run my Forester at ~ 2 psig
over the door. One reason manufactures pick the pressure is to provide
the driver with some advanced warning when the tire is starting to
lose traction. To date Ford is the only big screw-up by recommending a
pressure lower than their own and Firestones engineers recommendation
on the Explorer and we know what happened to the rolly polly Ford when
a blowout occurred.

 

[Tcassette]

I have Goodyear Triple Treads on my 96 Outback, at about 29 psi. They don't
appear underinflated, and the ride and handling is fine.

 

[BobN]

Currently I'm running the tires at 36 psi, and they feel fine to me. A
firmer feel with easy rolling is more to my liking, if I wanted a cushy ride
I'd have kept my Cadillac. I'm going to try emailing Goodyear and will
report back if I get a useful reply.

 

[Edward Hayes]

I suspect the dealer fill the tires during the hot part of the day. I
would refill the tires after sitting overnight using a good gage like
a digital.

 

[Yousuf Khan]

Last Friday I put a set of Goodyear Comfort Treads on my 98 OBLW. The tire
dealer filled them to 29 psi as spec'd on the door sticker, but the tires
looked really flat at that pressure. The maximum recommended pressure on
the sidewall is 44 psi but the dealer said that would completely wrong for
the car. That got me to thinking.

About a year or two back I ran my tire pressure right at the 44psi upper
limit, because I had found some of my tires had lost their shape after I
went out of the country for a month. I was trying to use the higher
pressure to reshape them back to shape. I didn't find that the tires
were any less grippy at maximum pressure either on dry or wet roads.
Mind you, I hardly take them to their limits most of the time, but
neither do most people.

Yousuf Khan

 

[Phil]

I don't know of any method for calculating tyre pressure ratios. However, I
experienced severe wear in the Yokohamas originally supplied with my 05 OBW.
I too was using the recommended tyre pressure until someone in this group
suggested increasing. I did this up to almost maximum, less about 5 psi and
the wear in the tyres has decreased markedly. I don't explain 'em, I just
does it!!! Good luck.

 

[Googler]

I thought it was common knowledge that as long as the tires you are
replacing are the SAME SIZE as the new ones, the automobile
manufacturer's inflation recommendations are the ones to use. The car
maker's engineer's have determined based on the best combination of
handling, fuel economy, ride, tire wear, etc.. what those inflation
pressures should be. Sure, one can inflate lower or higher to suit
one's taste but remember this: someone who's sole job was to make that
determination probably went to a lot of trouble to come up with the
right number.

 

[Dominic Richens]

[...] That got me to thinking.

Famous last words - 2nd only to "Hey watch this..."

I reckon that the tire is designed to hold a certain shape under
load, and depending on the weight of the car that would involve
different pressures for different cars, and the maximum on the
sidewall would refer to the pressure at the highest rated load.

The maximum mainly referes to bead tension - i.e. the maximum pressure at
which the tire will reliably stay on the rim.

Different tires also have different sidewall stiffnesses, so it
doesn't make sense to go by the OEM door sticker when changing to a
different brand/model tire.

You are forgetting that 99.99...% of the weight of the car is supported by
the air in the tire. The side walls do little beside keeping the air in.
Hence tire air pressure is 99.99...% a fuction of vehicle weight, not tire
construction. The only effect that tire construction will have is in the
volume of air containted, which should be constant for all tires of a given
size.

If I put too much pressure in the tire
it will have the wrong shape and not handle correctly.

You are assuming that you know what shape the tire designer intended the
tire to be in.

cheers!

 

[M. Brumbaugh]

I've got the Goodyear Triple Treads on my 04 Forester and I run them at ~
34-36psi. This is a good compromise between fuel economy and comfort. The
lower oem suggested pressure would give you greater comfort (ie: not as
jittery) over bumps etc, but sacrifices handling and fuel economy. Greater
than 40psi would probably be too coarse of a ride for most drivers. I assume
that quiet and comfort are important to you based on your tire choice. Try
34psi and adjust from there. You'll know when it's too much when the wheels
feel like solid rubber over railroad tracks etc.

I just put Triple Treads on my '03 WRX Wagon. Recently checked my
tire pressure. Finding them around 35 psi, which would have been done
at install, I filled them to 40 psi (they max at 44 psi) and I'm much
happier with the ride and the handling.

YYMV

MB

 

[JD]

I just put Triple Treads on my '03 WRX Wagon. Recently checked my
tire pressure. Finding them around 35 psi, which would have been done
at install, I filled them to 40 psi (they max at 44 psi) and I'm much
happier with the ride and the handling.

You won't be much happier with the tire wear at 40 PSI

 

[JD]

Last Friday I put a set of Goodyear Comfort Treads on my 98 OBLW. The
tire
dealer filled them to 29 psi as spec'd on the door sticker, but the tires
looked really flat at that pressure. The maximum recommended pressure on
the sidewall is 44 psi but the dealer said that would completely wrong for
the car. That got me to thinking.

I reckon that the tire is designed to hold a certain shape under load, and
depending on the weight of the car that would involve different pressures
for different cars, and the maximum on the sidewall would refer to the
pressure at the highest rated load. Different tires also have different
sidewall stiffnesses, so it doesn't make sense to go by the OEM door
sticker
when changing to a different brand/model tire. If I put too much pressure
in the tire it will have the wrong shape and not handle correctly. But I
like to have the highest reasonable pressure to decrease rolling
resistance,
since most of my miles are highway.

Is there a way to calculate the correct pressure for a specific tire and
car
weight (assuming typical passenger/cargo load and driving conditions)?

The OEM tire pressures are based on the OEM tire ratiing (ie. S, H, etc) and
the weight and balance of the car and suspension. If you are going to a
higher speed rating, the sidewall will be stiffer and you can get away with
lower or slightly greater pressure.

Just remember that too low pressure is a problem because of heat build-up.
Higher pressure deforms the tire laterally putting more pressure on the road
in the centre of the tire. That will cause the tire to fail quicker,
because you may have plenty of tread on the edges, and be to the belts in
the middle. The tire should look slightly flat and the contact patch should
be as flat as possible longitudinally as well as laterally. If you over
inflate them, you actually reduce the size of the contact patch and your
traction as well. If you want to inflate them to high pressure (for
handling), you are better off getting a bigger wheel with a lower profile
that increasing the tire pressure.

 

[Rick Courtright]

one's taste but remember this: someone who's sole job was to make that
determination probably went to a lot of trouble to come up with the
right number.

Hi,

As a normal practice with things automotive, I've learned the engineers
generally know what they're talking about, and most attempts to
"improve" upon something may actually improve one aspect of performance,
while compromising another. Unfortunately, the engineers are tasked with
designing a product that makes the most people happiest the most time.
Which means there's always gonna be something that each of us as
individuals will like or dislike more than another might.

Which leads us to tires...

For my own uses, I've always found it's beneficial to use the door
sticker as the "minimum" and some door stickers allow more leeway than
others. For example, my older Subie says 28/28 psi frt/rr w/ light
loads, 28/32 psi frt/rr w/ full loads, yet the gross axle weights are
calculated at 36/36 psi frt/rr. So there's some room for experimentation
in there, and I've found 32/32 to 34/32 frt/rr gives me the best combo
of feel, fuel economy and tire mileage under MY driving conditions.

Even discounting our individual driving differences, there's
temperature. Some areas experience rather constant temps for reasonable
time periods. But here in the part of SoCal where I am, it can easily be
35 deg F one morning, 55 or 60 the next. So if one inflates the tires to
the door figure on a warmer morning, the next morning they could be
"low." To help compensate, I've been told to "always" go at least 2 psi
over the door sticker as a "starting point" by numerous tire dealers
over the years, and, like Ed H, find that good advice. Then one can fine
tune as desired.

Regardless, I'm in agreement with other posters a GOOD gauge is
mandatory! I've used digitals and have an Accutire, which is a nice one,
but prefer the Accu-Gage analog cuz it's easier to bleed off excess
pressure and match the tires. Remember, too, even the best guages have a
little leeway (+/- 1 or 2% and up depending on pressure) in their
accuracy--another reason to go a bit over sticker pressure.

Rick

 

[CL (dnoyeB) Gilbert]

I thought it was common knowledge that as long as the tires you are
replacing are the SAME SIZE as the new ones, the automobile
manufacturer's inflation recommendations are the ones to use. The car
maker's engineer's have determined based on the best combination of
handling, fuel economy, ride, tire wear, etc.. what those inflation
pressures should be. Sure, one can inflate lower or higher to suit
one's taste but remember this: someone who's sole job was to make that
determination probably went to a lot of trouble to come up with the
right number.

if your using the tires the engineers designed too, and using the rims
the engineers designed too, and suspension, etc., then yes. If you
start changing things then no.

--
Thank you,


CL Gilbert
"Then said I, Wisdom [is] better than strength: nevertheless the poor
man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16

 

[y_p_w]

if your using the tires the engineers designed too, and using the rims
the engineers designed too, and suspension, etc., then yes. If you
start changing things then no.

Still - it's usually a one size fits all proposition. I filled my 2004
WRX tires to 4-5 PSI over the door jamb numbers with Pirelli P
Zero Nero M+S tires. I'm finding that there seems to be a bit
more wear down the center of the tires. I'm trying a little bit less -
maybe 2 PSI over the factory recs.

The problem is that the number on the door jamb is a compromise.
It won't always account for different tires of the same size, load,
etc. Too little pressure and the tires run hot. Too much pressure
actually makes handling "jittery". It's really a starting point more
than anything else.

Also - there seems to be an assumption that the number was
precisely determined. I find that sometimes engineers come up
with an acceptable range, but then marketing takes over to try
to make the product more palatable.

 

[Pinehollow]

Hi,

As a normal practice with things automotive, I've learned the engineers
generally know what they're talking about, and most attempts to
"improve" upon something may actually improve one aspect of performance,
while compromising another. Unfortunately, the engineers are tasked with
designing a product that makes the most people happiest the most time.
Which means there's always gonna be something that each of us as
individuals will like or dislike more than another might.

Which leads us to tires...

For my own uses, I've always found it's beneficial to use the door
sticker as the "minimum" and some door stickers allow more leeway than
others. For example, my older Subie says 28/28 psi frt/rr w/ light
loads, 28/32 psi frt/rr w/ full loads, yet the gross axle weights are
calculated at 36/36 psi frt/rr. So there's some room for experimentation
in there, and I've found 32/32 to 34/32 frt/rr gives me the best combo
of feel, fuel economy and tire mileage under MY driving conditions.

Even discounting our individual driving differences, there's
temperature. Some areas experience rather constant temps for reasonable
time periods. But here in the part of SoCal where I am, it can easily be
35 deg F one morning, 55 or 60 the next. So if one inflates the tires to
the door figure on a warmer morning, the next morning they could be
"low." To help compensate, I've been told to "always" go at least 2 psi
over the door sticker as a "starting point" by numerous tire dealers
over the years, and, like Ed H, find that good advice. Then one can fine
tune as desired.

Regardless, I'm in agreement with other posters a GOOD gauge is
mandatory! I've used digitals and have an Accutire, which is a nice one,
but prefer the Accu-Gage analog cuz it's easier to bleed off excess
pressure and match the tires. Remember, too, even the best guages have a
little leeway (+/- 1 or 2% and up depending on pressure) in their
accuracy--another reason to go a bit over sticker pressure.

Rick

I have three tire gauges, one digital and 2 analog. I check the tires in
the garage, cold, about every two weeks. The first tire, I use all three
gauges to make sure that they all agree. The rest of the tires on both
vehicles I just use the digital gauge because it is easier. For the last
several years, all three gauges have agreed, so I have no problem. I still
haven't decided what I will do when they disagree. It depends on whether
two agree and one disagrees, or if all three have different pressures. I'll
decide when that day comes.

Don

 

[TBerk]

What this guy said.

The sticker is for the tires the car came with. How can they apply that
psi to a tire which came out later? What form of time travel do the
engineers have, that they can fortell what tires are like years later?

Compare Max psi ratings on the old and new tires. Are they the same
upper limit? No?

Hmmmm....


TBerk

 

[CL (dnoyeB) Gilbert]

I have three tire gauges, one digital and 2 analog. I check the tires in
the garage, cold, about every two weeks. The first tire, I use all three
gauges to make sure that they all agree. The rest of the tires on both
vehicles I just use the digital gauge because it is easier. For the last
several years, all three gauges have agreed, so I have no problem. I still
haven't decided what I will do when they disagree. It depends on whether
two agree and one disagrees, or if all three have different pressures. I'll
decide when that day comes.

Don

ROFL

--
Thank you,


CL Gilbert
"Then said I, Wisdom [is] better than strength: nevertheless the poor
man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16