Question about tires pressure for cars that rarely

[MetalSlug]

Question about tires pressure for cars that rarely drive.

Just got 4 brand new tires put on my stock mustang 15 inch wheel, I using the factory size 225/60/15 , the guy install my tires put in 38 PSI ( I bring in 4 wheels and let them put in 4 tires on it, I dont drive my mustang in )

anyway I drop down to 35 psi . factory spec is 30 psi. for right now, I TRY to drive it 1-2 time a week on long distant ( 30-40 miles driving ), I know tires psi drop when you let your car sit in storage for 1+ month. thank you

another question is, when tires psi usually drop most in winter or summer ?

 

[ExMachina]

Tires vary widely in how much air they would lose just sitting around. Depends on how well they seal, and every tire is a little different on that. I would expect new tires to not lose much at all though. Hope for that anyway. I would say tires lose more air in winter since the seals/rubber is harder and contracts a bit.

 

[Quattro Pete]

Supposedly higher pressure helps prevent flatspotting, but I haven't really seen this to be the case from my own personal experience.

If the label says "30" then that's probably where I would start, and if it doesn't feel right, go up from there.

Quote:
another question is, when tires psi usually drop most in winter or summer ?

Tires lose some pressure throughout the year, regardless if winter or summer. But you will notice some pressure drop when going from summer to winter, just due to ambient temperature drop. For every 10 degree F drop in temperature, your pressure will be 1 PSI lower. Of course when the temperature goes back up, so will your pressure.

 

[MetalSlug]

Thank you guys, very nice info.

the label on tires max psi is 44. Will it hurt the tires if I running 35 psi.

I also heard, that factory set PSI spec on each car, because it help the tire wear out evenly , by pump up the tires above factory spec made the tires wear out unevenly...

I dont know if this true or not.

 

[Quattro Pete]

Originally Posted By: MetalSlug
the label on tires max psi is 44. Will it hurt the tires if I running 35 psi.

It shouldn't.

Quote:

I also heard, that factory set PSI spec on each car, because it help the tire wear out evenly , by pump up the tires above factory spec made the tires wear out unevenly...

It really depends on a particular car and maybe even on a particular tire. If you only go a little above factory PSI, it shouldn't hurt anything. Some people will significantly increase their pressure above factory spec in order to achieve better MPG. Personally, I just stick with factory PSI because I like the balance between comfort and steering response, and my tires do wear out the most evenly then.

 

[HTSS_TR]

Originally Posted By: MetalSlug
The label on tires max psi is 44. Will it hurt the tires if I running 35 psi.

No, it doesn't hurt the tires if you're running few psi above recommended and below max pressure on tire's sidewall.

Originally Posted By: MetalSlug
I also heard, that factory set PSI spec on each car, because it help the tire wear out evenly , by pump up the tires above factory spec made the tires wear out unevenly...

Usually tire pressure on car placard is for a softer ride, soft ride car is usually sell easier. For better performance/handling and fuel economy and longer tread life 2-4 PSI above placard will do that, but the ride will suffer a little.

 

[Wolf359]

Originally Posted By: MetalSlug
another question is, when tires psi usually drop most in winter or summer ?

Have we learned nothing from deflategate? Or at least Boyle's law? The pressure will drop with temperature. Assuming you're not losing any air, the pressure will be lower in the colder winter temperature.

Running 35 is probably fine. I typically run 35 on my Taurus in the front and about 32 in the rear although the car calls for 30. 30 is the softer ride, Ford is well known for specing a lower pressure for a smoother ride. That's what turned the Ford Explorer into the exploder, tire pressures were speced low to begin with for a smoother ride and then when it dropped even lower due to lack of maintenance, additional flex lead to tire failure.

 

[Revving2Redline]

Run N20 in your tyres and forget about it. I have used nitrogen from Quicklane and my tyres wear better and last longer than they should. The ride is better also. My car sits out all winter and with nitrogen the tpms never flashed low air.

 

[Johnny2Bad]

It's true that Nitrogen leaks less than air, plus N displaces moisture. As for winter, although again it's true temperature affects pressure, cold also causes more leakage.

Weekly pressure checks will show you will fill more often in winter, even if cold temps are stable.

 

[m37charlie]

Originally Posted By: Quattro Pete
For every 10 degree F drop in temperature, your pressure will be 1 PSI lower. Of course when the temperature goes back up, so will your pressure.


Not exactly. The universal gas law is PV = nRT. Assuming 50 F ambient, that is 503 absolute (F degrees). 10 degrees is 2%. For P one must take measured pressure and add 1 atm~15psi. There is of course gas at 1 atm in a "0 psi" tire - same as outside the tire. So for a 35 psi tire at 50 F your statement is approximately true.
But for a 100 psi tire (+15 for "zero inflation"), 10F rise gives 2.3 psi increase.
Believe me, I've had a lot of time to figure this out staring at the PSI display on the CTIS part of my dash on my Unimog.

Charlie

 

[CapriRacer]

A couple of thoughts:

A tire grows over its life - especially in the first 24 hours. That makes the volume in the air chamber larger and reduces the pressure. In our test garage, the guys used to add 3 psi to the desired pressure when the tires were new (except when they were going to immediately test those tires).

Second, while tires do leak air, if everything is sealed up correctly between the tire and the wheel, there should be very little pressure loss over time - maybe a psi a month. - BUT - it would be a good idea to check this early to establish that there is a good seal!

In a vehicle that sits a lot, more pressure will help prevent flatspots. Also moving the vehicle helps (or more correctly, creates a bunch of small distributed flatspots, rather than one big one!)

And a bit more pressure in a vehicle isn't a really big deal. Certainly given the situation in this thread, 5 psi above the recommendation is a good number.

And cold temperatures will cause the pressure to go down - about 1 psi for every 10°F. Plan accordingly.

 

[KitaCam]

+1 for all the above by Capri
...as Capri writes, and read elsewhere, 1-lb loss per month is acceptable, my experience is also as he writes....my alloys do not lose any psi for months...and I've left the car sitting for 3 weeks without any pressure loss.

 

[JHZR2]

Tires have two things going on. Leakage/seepage and thermal issues.

Leakage is tire/wheel/valve stem dependent and will vary slightly by tire. But it should be reasonably consistent once you understand rates.

Thermally, pressures go up when warmer, down when colder. That's part of the reason why it's best to check pressure in tires that have sit and not been run or in direct sun for a while.

I'd run my tires for lomg-sitting cars as high as is practical/comfortable. Many of our newer cars are asking for 38-41 psi in normal use, and I'd guess that 35-40 in your tire is ok...

 

[Quattro Pete]

Originally Posted By: Wolf359
Ford is well known for specing a lower pressure for a smoother ride. That's what turned the Ford Explorer into the exploder, tire pressures were speced low to begin with for a smoother ride

Actually, in case of the Explorer, Ford specced lower pressure not because they wanted a smoother ride but because they were hoping it would reduce the chance of rollovers.

 

[ExMachina]

Originally Posted By: Quattro Pete
Originally Posted By: Wolf359
Ford is well known for specing a lower pressure for a smoother ride. That's what turned the Ford Explorer into the exploder, tire pressures were speced low to begin with for a smoother ride

Actually, in case of the Explorer, Ford specced lower pressure not because they wanted a smoother ride but because they were hoping it would reduce the chance of rollovers.


I remember that! .NY Times article says they WERE at a very low 26 psi, a bomb waiting to go off in high speed hot weather conditions in some cases.
"DETROIT, Sept. 22, 2000— The Ford Motor Company recommended tonight that owners of Ford Explorer sport utility vehicles inflate their tires to 30 pounds per square inch, instead of the 26 pounds that the automaker has recommended for the last 10 years."

 

[George Bynum]

Originally Posted By: m37charlie
Assuming 50 F ambient, that is 503 absolute (F degrees).Charlie
It's insignificant, but I believe 50F would be 513R. "absolute zero" is about -460F.

 

[Snoman002]

Just remember, the factory sticker on the door jam is for factory tires with a factory load range. It's highly unlikely there "best" pressure would be wildly different in this case, but something to be aware of.

Personally I'm a fan of measuring the contact patch to determine proper pressure.

 

[CapriRacer]

Originally Posted By: Snoman002
Just remember, the factory sticker on the door jam is for factory tires with a factory load range. It's highly unlikely there "best" pressure would be wildly different in this case, but something to be aware of.

Personally I'm a fan of measuring the contact patch to determine proper pressure.
A couple of thoughts:

1) The vehicle tire placard will list the original tire size and the specified pressure for that SIZE. Please note that for passenger cars, this pressure would be the same REGARDLESS of who manufactures the tire. That's because tires are standardized. I go into this in much greater detail here:

Barry's Tire Tech - Load Tables

For pickup trucks and vans, the situation is a bit more complex. P type tires are load rated one way, and LT tires are rated another way. So if your pickup or van came with P type tires and you want to put on LT tires, you will need to adjust the pressure.

If your vehicle came with LT tires, then you can NOT safely put on P type tires.

2) Measuring the contact patch is NOT a good way to decide what the inflation pressure should be. I do not have any idea HOW that could be done, but ALL the tire manufacturers use the load vs pressure charts to test the durability of their products - and NONE of them use the contact patch size or shape as a determining factor. They might measure the contact patch in order to provide a better wearing or better handling product, but that is a consequence of the design, not a criteria for durability or ride.

3) The Ford Explorer situation a few years back: I go into that in detail here:

Barry's Tire Tech - The Ford/Firestone Controversy

Short version: While Ford's spec for the inflation pressure for the Explorer was not the best, it wasn't the real problem. Ford, GM, Dodge, all specified pressures the same way - the load carrying capacity of the tires = GAWR (Gross Axle Weight Rating). Not good engineering practice - BUT - they all did it, and only the Ford Explorer with the Firestone tires had issues. Even Ford Explorers with other brands did not have issues.

The problem was a combination of a quirky tread pattern and an odd manufacturing technique. Both of those things have been identified by both Firestone and other tire manufacturers and appropriate changes have been made. The fact that Ford increased the inflation pressure specification is more a reflection of recognizing there was a BETTER way to specify the load carrying capacity of the tires.

 

[ecotourist]

Originally Posted By: m37charlie
The universal gas law is PV = nRT.


Absolutely right.

From memory: P is pressure. V is volume. n is the number of moles of gas. R is the universal gas constant. And T is the temperture in absolute degrees.

Let's assume the volume stays constant (that's close to true). If there's no leakage, n is a constant. And R is a constant. Let's also assume there's no water, which would complicate things.

So, rearranging the terms: P/T = nR/V = a constant.

Therefore, P1/T1 = a constant = P2/T2

You determine T in degrees Rankin (an absolute temperature) by adding 460 to temperature in degrees F. Or you determine T in degrees Kelvin (also an absolute temperature) by adding 273 to temperature in degrees Celsius.

Rearranging the terms again: P2 = P1 ( T2/ T1)

Suppose your tires have 30 psi gauge pressure (plus 14.7 psi to get 44.7 psi absolute) at 70 degrees F (= 530 degrees R). So what's the pressure at 30 degrees F (= 490 degrees R)?

P2 = 44.7 psi absolute pressure times (490/530)= 41.3 psi absolute - 14.7 psi = 26.6 psi gauge pressure.

Assuming my math is correct (and someone should check it), a tire that was at 30 psi at 70 degrees would have 26.6 psi at 30 degrees.

 

[CapriRacer]

Originally Posted By: ecotourist
Originally Posted By: m37charlie
The universal gas law is PV = nRT.


Absolutely right.

From memory: P is pressure. V is volume. n is the number of moles of gas. R is the universal gas constant. And T is the temperture in absolute degrees.

Let's assume the volume stays constant (that's close to true). If there's no leakage, n is a constant. And R is a constant. Let's also assume there's no water, which would complicate things.

So, rearranging the terms: P/T = nR/V = a constant.

Therefore, P1/T1 = a constant = P2/T2

You determine T in degrees Rankin (an absolute temperature) by adding 460 to temperature in degrees F. Or you determine T in degrees Kelvin (also an absolute temperature) by adding 273 to temperature in degrees Celsius.

Rearranging the terms again: P2 = P1 ( T2/ T1)

Suppose your tires have 30 psi gauge pressure (plus 14.7 psi to get 44.7 psi absolute) at 70 degrees F (= 530 degrees R). So what's the pressure at 30 degrees F (= 490 degrees R)?

P2 = 44.7 psi absolute pressure times (490/530)= 41.3 psi absolute - 14.7 psi = 26.6 psi gauge pressure.

Assuming my math is correct (and someone should check it), a tire that was at 30 psi at 70 degrees would have 26.6 psi at 30 degrees.



- OR -

You could use the rule of thumb: 1 psi change for every 10°F.