Author Topic: Tire pressure  (Read 14476 times)

Hutch

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Tire pressure
« on: September 05, 2011, 01:53:35 PM »
Sure seems like this would be straightforward, but noooooooooo.....

I have asked 2 different authoritative sorts, and got 2different answers.

Should I inflate the tires on my car to the max pressure shown on the sidewall, or to the pressure shown on the medallion on the door frame of the car?  Why?
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Monkeyleg

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Re: Tire pressure
« Reply #1 on: September 05, 2011, 02:11:28 PM »
Are the tires the original brand and model # that came with the car when new? If so, then use the pressure listed on the door plate. If not, then inflate to the maximum pressure shown on the tire, or a bit less. Most car tires are inflated to 32 to 35 pounds (you didn't say if it was a car or truck tire). The maximum pressure will give a harder ride than the slightly lower pressure.

Adjust pressure when the tires are cold, as the pressure increases as the tires heat up while driving. I hate it when I go to a quick change oil place and one of the "technicians" lets air out of my tires because he says the pressure is too high. It's not too high, it's normal pressure after having been driven for more than 10 miles.

Doggy Daddy

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Re: Tire pressure
« Reply #2 on: September 05, 2011, 02:12:04 PM »
Sure seems like this would be straightforward, but noooooooooo.....

I have asked 2 different authoritative sorts, and got 2different answers.

Should I inflate the tires on my car to the max pressure shown on the sidewall, or to the pressure shown on the medallion on the door frame of the car?  Why?

I read something authoritative a while back that said to go by the medallion.  But I forget the reason behind it.  I'm thinking it's because the rating on the tire represents the max that the tire should be inflated to under any condition, but the figure on the medallion is the inflation for the condition of that particular vehicle's weight and handling characteristics.

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Re: Tire pressure
« Reply #3 on: September 05, 2011, 02:27:07 PM »
This is car and not truck? On my F250, the door panel calls for 60F/75R. If I do that when there's no load in the back, it's bounce city, so I keep everything at 60. On my trooper, the door panel is 35F/30R and I use that with good results.
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Hutch

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Re: Tire pressure
« Reply #4 on: September 05, 2011, 02:33:50 PM »
2007 Camry.  Bought used ~ 45kmi. Tire says max load at 50 psi, but medallion reads in the 30's, iirc.  That is a mighty wide spread.

Thanks, keep 'em coming.
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Brad Johnson

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Re: Tire pressure
« Reply #5 on: September 05, 2011, 02:44:21 PM »
The car maker's tire pressure recommendation is made from long-term testing and is the best compromise of ride quality, economy, and wear characteristics.  The pressure on the tire sidewall is the tire manufacturer's maximum "do not exceed" pressure and is there strictly for safety reasons.

For economy reasons I typically run my tires at about 10% over the mfg recommendation.  Any more, especially on a light-in-the-behind truck, and traction becomes an issue, especially in the wet.

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birdman

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Re: Tire pressure
« Reply #6 on: September 05, 2011, 05:28:52 PM »
Follow the medallion.  As stated before, the sidewall number is the maximum allowable pressure...and since temperatures can vary pressure substantially, if you filled to that pressure cold...and had some moisture in the tire from periodic fills with humid warm air, and venting when cold out...you could easily blow the tire the next day if it was hot.  Best thing to do is to fill to medallion, and if you really want to push it, fill to medallion, then drive it hard, on a hot day, under heavy load, and check the pressure hot...if it's still substantially below the sidewall...you could go up a bit if you want to get better mileage.

zxcvbob

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Re: Tire pressure
« Reply #7 on: September 05, 2011, 05:30:48 PM »
The pressure on the door is the minimum you should use (assumes original size tire.)  The number on the tire sidewall is the maximum.  Pick a number you like that's in between.  Also, when you get your oil changed it's a good idea to tell them not to mess with your tire pressures, otherwise some will let the air out to get it down to what the door plaque says -- even if you have a totally different size tire.

Don't go over 65# if you have rubber valve stems.
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Azrael256

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Re: Tire pressure
« Reply #8 on: September 05, 2011, 05:57:06 PM »
Quote
As stated before, the sidewall number is the maximum allowable pressure

It is the maximum pressure for the load range.  The bead seating pressure is the maximum allowable before the sidewall goes.  The trick is that tire pressure is dynamic.  You measure it at 60F at 1200ft with 25% humidity and it's *this*.  Drive all your friends down to the beach at 90F and run over a speedbump too fast and it will be entirely different.  If you stay on top of it, that might work.  If you inflate it to the 44psi on the sidewall, you may find it dangerously close to blowing out when conditions change.  The manufacturer has (with the notable exception of Ford) generally done the math (and hopefully testing) to give you a number that will not allow you to cross the minimum/maximum under normal conditions.

Quote
when you get your oil changed it's a good idea to tell them not to mess with your tire pressures
I always asked customers how far they had driven to get to me and only really made an effort to inflate an obviously low tire.  If it was cool out and they had just driven around the block to get to me, I'd set it to the manufacturer's recommendation.  After more than maybe 5 miles, or if it was a hot afternoon, you can't tell.  On a warm afternoon, I could tell a customer what direction their car had been facing while parked by the tire pressures.  You could have one side sitting at the specified 32# and the other side at 38# if they had parked it right.

I check mine in the morning while still in the garage, and I'll usually overinflate by less than 10%, but I do exceed the sticker pressure on most vehicles.  Except Ford SUVs (which we called "flippers").

I did have a few come in with 12#.  I don't know how a tire stays on the rim at that pressure, but they did.  Also, it's all different with low-profile tires.  They'll pick up the same heat from the road surface, but far less solar gain.  They also have a lower volume, so their pressure drops faster.

If you really cared all that much, you'd run the tire over a wet paint stripe at varying pressures to find the largest contact patch that stayed on the tread.  That's probably going to be the "best" inflation level.  The thing is that there is no magic number.  Overinflated tires have lower resistance so you'll improve your mileage.  You'll be riding on a thin strip of rubber, so you'll lose handling and tread life.  At current prices, halving your tread life might actually be worth it, but do that in the rain and you yourself might live half as long.

ETA: You will need a MAJOR change in tire size (like putting gigantic mud tires on your Jeep) to change the optimum pressure by more than the margin of error on your gauge.  If you change the load range of the tire, then you'll throw the pressure to something completely different.
« Last Edit: September 05, 2011, 06:21:30 PM by Azrael256 »

sanglant

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Re: Tire pressure
« Reply #9 on: September 05, 2011, 06:13:04 PM »
the max, is the max cold pressure. tires heating up is taken into account in setting them. ;)


now then, there are two ways to find the right pressures for your car and tires. first up we have the math. you take the wet weight of the car, front and back. and the max tire pressure, and max weight and figure out what percentage of the max you need. :laugh:

next up, you fill 'em to the door sticker. no that ain't it. >:D find a place you can drive 10-20 feet or so and stop, 10 to 20 times. chalk the thread from the inside sidewall to the outside, at least one front tire and one back tire. better to do both sides. if the center rubs off faster, pressure is to high. the edges rub off faster, pressure is to low. ;)

oh, and running higher pressures lowers the temps your tires are reaching. and gives you better mileage, but can make the car slide around more. on the other hand you'll be hydroplaning less. [popcorn]

zahc

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Re: Tire pressure
« Reply #10 on: September 05, 2011, 11:53:45 PM »
This discussion has been fun, but let's take it up a notch.

Is there a difference if the mechanic checks the tire pressure when the car is on the lift, with no weight on the tire, versus checking the tire pressure when the car is sitting on the tire? What's the difference? Which is the correct way?
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Azrael256

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Re: Tire pressure
« Reply #11 on: September 06, 2011, 12:10:02 AM »
There is a difference, but your average tire gauge won't detect it.  Lowering the car onto its wheels deflects the tire a bit, but not enough to noticeably decrease volume.

CNYCacher

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Re: Tire pressure
« Reply #12 on: September 06, 2011, 12:18:09 AM »
There is a difference, but your average tire gauge won't detect it.  Lowering the car onto its wheels deflects the tire a bit, but not enough to noticeably decrease volume.

Then what holds the car up?
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HankB

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Re: Tire pressure
« Reply #13 on: September 06, 2011, 05:56:43 AM »
Some years ago when Ford was having problems with Firestone tires on Explorers, the medallion pressure was, IIRC, 26 PSI. The temporary "fix" was to increase pressure to 30 PSI, which is where I had been keeping it all along. Sidewall pressure was 35 PSI.

Never exceed the sidewall pressure!

I normally keep my tires a couple of PSI above the medallion, which theoretically reduces ride comfort but improves mileage. Pressure is still well below the sidewall max. Keep an eye on the wear pattern - if your tread starts showing less depth down the middle, you're probably overinflating.
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Re: Tire pressure
« Reply #14 on: September 06, 2011, 08:27:49 AM »
When measuring a cold tire, I like to stay a few pounds under the sidewall pressure (35psi = 30-32psi for me)....gives it plenty of room for heat expansion....
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zahc

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Re: Tire pressure
« Reply #15 on: September 06, 2011, 09:38:32 AM »
Quote
Keep an eye on the wear pattern - if your tread starts showing less depth down the middle, you're probably overinflating.

With the new low-profile tires, the opposite can also be the case. The wide, narrow-sidewall tires need pressure to hold shape, and underinflating can cause the center to wear faster as the tire is deformed by centrifugal force (so I've been told).

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Re: Tire pressure
« Reply #16 on: September 06, 2011, 10:43:06 AM »
If your vehicle is AWD stick to the medallion and stick to OEM tire size, religiously.
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brimic

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Re: Tire pressure
« Reply #17 on: September 06, 2011, 10:57:58 AM »
Quote
when you get your oil changed it's a good idea to tell them not to mess with your tire pressures with anything except the oil and filter
FIFY.

I go a few PSI under the max pressure on the tire sidewall- this has never caused a problem.
I yelled at the quickielube monkey once who was letting the air out of my tires to bring them down to 33 PSI when the pressure was at 38 and the sidewalls said 41.
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sanglant

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Re: Tire pressure
« Reply #18 on: September 06, 2011, 11:09:26 AM »
If your vehicle is AWD stick to the medallion and stick to OEM tire size, religiously.
better buy back the same tires to then, and good luck with that, here's a link to the tires that came on my car. there's no way in hell i'm paying over four hundred bucks for tires that roll onto the sidewalls at the specced pressures. it's really easy to find out the right pressures. being lazy is just going to cost you. might even be your life, or worse. having to live with killing some kid. =|

zxcvbob

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Re: Tire pressure
« Reply #19 on: September 06, 2011, 11:55:00 AM »
better buy back the same tires to then, and good luck with that, here's a link to the tires that came on my car. there's no way in hell i'm paying over four hundred bucks for tires that roll onto the sidewalls at the specced pressures. it's really easy to find out the right pressures. being lazy is just going to cost you. might even be your life, or worse. having to live with killing some kid. =|
I would probably switch those to 195/65/15's.  You can get H-rated tires with A A temperature and traction ratings for about $65 to $75 each.
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sanglant

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Re: Tire pressure
« Reply #20 on: September 06, 2011, 12:12:03 PM »
205/60R15 isn't really a problem size, i'm running much better/cheaper tires. =) that was just what it came with. and tehy were ok for learning the cars limits* but that was done within 3000 miles. >:D


*they would start screaming like banshees under 15mph in moderate curves and you could push well past that point before they started slipping but they also rolled onto the side walls before they started sliping. [tinfoil] looked almost like bike tires when i was done with 'em. >:D

Brad Johnson

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Re: Tire pressure
« Reply #21 on: September 06, 2011, 12:39:03 PM »
The mfg maximum on the sidewall takes into account potential pressure increases due to heating.  That's why the rating is given along with a temp (i.e. 44 psi at 70 deg F).

Brad
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sanglant

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Re: Tire pressure
« Reply #22 on: September 06, 2011, 06:08:57 PM »
a few of us have said that already. ;) have we missed any more ways to say it? :laugh:

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Re: Tire pressure
« Reply #23 on: September 06, 2011, 06:44:05 PM »
Don't forget you have change them from winter to summer air and vice versa when the seasons change. >:D
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Tallpine

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Re: Tire pressure
« Reply #24 on: September 06, 2011, 07:01:08 PM »
If your vehicle is AWD stick to the medallion and stick to OEM tire size, religiously.

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