tyre pressure question

[Maylix]

I'm used to tyre pressures going down due e.g. to slow puncture etc, but does DAK, if its possible to pump a tyre up to say 30 psi, and then a few days later the pressure in the tyre be measured at 40 psi? (assuming nothing wrong with the pressure measuring equipment)
TIA
Maylix

[Spet0789]

Pressure will rise in proportion to temperature but a pressure change of that magnitude isn’t possible.

If you put air in your tyres on a cold U.K. day (say -3 degrees C) and then the temperature magically went up to 27 degrees C, and then you drove like a lunatic and got the air in your tyres up to 57 degrees C, then your temperature change would be 330/270 -1 = 18% (expressing the temperature in degrees Kelvin).

According to Gay-Lussac’s law, pressure and temperature are proportionate to each other, all else equal.

If you started with your tyres at 30psi, they would go up to just over 35psi, assuming a non-expanding tyre. In reality, the tyre would expand so the pressure rise would be less.

[vrdiver]

You mentioned "30psi" and "40psi". That suggests that the gauge might not be accurate, as you've got some very rounded figures there!

Is this a hypothetical question, or a real life observation?

VRD

[MonsterMork]

assuming nothing wrong with the pressure measuring equipment

In that case somebody else is putting more air in the tyres without your knowledge

MM

[88V8]

Well, if the valve is at the bottom of the tyre when you take the reading, it will show a higher pressure as there's more weight on it

V8

[bungeejumper]

Well, if the valve is at the bottom of the tyre when you take the reading, it will show a higher pressure as there's more weight on it

[Nimrod103]

I have always regarded tyre pressure gauges as rather unreliable. I have a couple of hand held gauges, plus the one on the pump, and one at the local service station.
I usually take half a dozen readings, out of which I usually find at least two measurements to be significantly low. I have always assumed it is because the valve can be a bit sticky.

[sg31]

My car has tyre pressure sensors that work in bars not psi. My tyre pressure when the car has been stood for a while has been very steady all summer at 2.3 bar. Now the mornings are cold the readings are 2.0 or 2.1. When I've been driving for a while the pressures increase to 2.2 or 2.3

I've got an electric tyre pump which shows the tyre pressure when cold at 2.2 or 2.3. When I drive to the local filling station the tyres have warmed up and are at pressure.

It's an irritating problem because the low tyre pressure warning takes out half of the info on the speedometer display and I have to reset it back to normal display every morning or everytime the car tyres cool down. I am considering increasing the tyre pressure slightly to compensate for the cold mornings but I'm not sure if that will mean I'm driving round with overpressured tyres most of the time.

[Padders72]

My car has tyre pressure sensors that work in bars not psi. My tyre pressure when the car has been stood for a while has been very steady all summer at 2.3 bar. Now the mornings are cold the readings are 2.0 or 2.1. When I've been driving for a while the pressures increase to 2.2 or 2.3

I've got an electric tyre pump which shows the tyre pressure when cold at 2.2 or 2.3. When I drive to the local filling station the tyres have warmed up and are at pressure.

It's an irritating problem because the low tyre pressure warning takes out half of the info on the speedometer display and I have to reset it back to normal display every morning or everytime the car tyres cool down. I am considering increasing the tyre pressure slightly to compensate for the cold mornings but I'm not sure if that will mean I'm driving round with overpressured tyres most of the time.

Not sure what vehicle you have, but on some if you reset the system when cold it will learn that you are happy with the 2.0 Bar cold pressure and not give you a warning until it senses a genuine pressure loss. Certainly on many BMWs you can do this. As you say if you overinflated your tyres so that you don't get the warning when cold, your running ie warm pressures will be sub optimal. Door plate pressures are often quoted for 20deg C, so will be 0.1-0.2 Bar less when cold. 1 Bar is circa 14.5psi btw.

[sg31]

Not sure what vehicle you have, but on some if you reset the system when cold it will learn that you are happy with the 2.0 Bar cold pressure and not give you a warning until it senses a genuine pressure loss. Certainly on many BMWs you can do this. As you say if you overinflated your tyres so that you don't get the warning when cold, your running ie warm pressures will be sub optimal. Door plate pressures are often quoted for 20deg C, so will be 0.1-0.2 Bar less when cold. 1 Bar is circa 14.5psi btw.

Thank you for that. I drive a VW Touareg. I will try resetting the system in the morning and see what happens. Maybe a pound or two overpressure might stop the warnings being triggered without too much deterioration in handling. I don't drive fast or corner hard so nothing is too critical, the vehicle is never near the edge.

[quelquod]

My car has tyre pressure sensors that work in bars not psi. My tyre pressure when the car has been stood for a while has been very steady all summer at 2.3 bar. Now the mornings are cold the readings are 2.0 or 2.1. When I've been driving for a while the pressures increase to 2.2 or 2.3

I've got an electric tyre pump which shows the tyre pressure when cold at 2.2 or 2.3. When I drive to the local filling station the tyres have warmed up and are at pressure.

It's an irritating problem because the low tyre pressure warning takes out half of the info on the speedometer display and I have to reset it back to normal display every morning or everytime the car tyres cool down. I am considering increasing the tyre pressure slightly to compensate for the cold mornings but I'm not sure if that will mean I'm driving round with overpressured tyres most of the time.

Not sure what vehicle you have, but on some if you reset the system when cold it will learn that you are happy with the 2.0 Bar cold pressure and not give you a warning until it senses a genuine pressure loss. Certainly on many BMWs you can do this. As you say if you overinflated your tyres so that you don't get the warning when cold, your running ie warm pressures will be sub optimal. Door plate pressures are often quoted for 20deg C, so will be 0.1-0.2 Bar less when cold. 1 Bar is circa 14.5psi btw.

In fact with the current system the BMW pressure monitor system measures both pressure and (tyre) air temperature and corrects for temperature. So if I look at my tyre pressure display the ‘recommended’ pressures vary depending on how warm a day it is and whether the tyres are warmer from running. I’m slightly amused on some sunny mornings when both the recommended settings and the actual pressures are substantially higher on the ‘sunny’ side of the car.

[Maylix]

Is this a hypothetical question, or a real life observation?

VRD

Thanks for all the replies guys. To answer VRDs question and explain a bit more, it's part hypothetical and part real life observation, viz:
I took my car to have a tyre repaired. The mechanic did the job, and then when I asked him what pressure he had inflated the repaired tyre to he said something ridiculous like 54psi ( the recommended is in the range 30-34psi) When I objected he said 'but it's a big tyre, it can take it'.
Unswayed by his erudite argument I asked him to reduce the pressure in that tyre and check the other ones to 32 psi. When I checked the pressures a couple of weeks later 3 tyres =32psi and the repaired one at 50oddpsi. So, as you guys didn't come up with any way that the pressure could have increased (I was thinking maybe a faulty vale, maybe losing a layer of rubber off the tyre??), the most likely cause was that the mechanic didn't reduce the pressure in the repaired tyre like I asked him to.
So Monster Mork gets the prize as he came nearest with his:
'In that case somebody else is putting more air in the tyres without your knowledge '
Thanks again everyone!

[swill453]

Unswayed by his erudite argument I asked him to reduce the pressure in that tyre and check the other ones to 32 psi. When I checked the pressures a couple of weeks later 3 tyres =32psi and the repaired one at 50oddpsi. So, as you guys didn't come up with any way that the pressure could have increased (I was thinking maybe a faulty vale, maybe losing a layer of rubber off the tyre??), the most likely cause was that the mechanic didn't reduce the pressure in the repaired tyre like I asked him to.

Also no evidence he even checked the other three...

Scott.

[jfgw]

I asked him what pressure he had inflated the repaired tyre to he said something ridiculous like 54psi ( the recommended is in the range 30-34psi) When I objected he said 'but it's a big tyre, it can take it'.

And I thought that, generally speaking, bigger tyres had less pressure. Broadly speaking. Roughly.

What is the maximum stated on the tyre?


Julian F. G. W.

[MonsterMork]

The mechanic did the job, and then when I asked him what pressure he had inflated the repaired tyre to he said something ridiculous like 54psi ( the recommended is in the range 30-34psi) When I objected he said 'but it's a big tyre, it can take it'.
Unswayed by his erudite argument I asked him to reduce the pressure in that tyre and check the other ones to 32 psi.

In which case never darken their doorway again as the "mechanic" is clearly a delicate anatomical area

We also fit tyres at the garage I work at and the only time a tyre is taken to 50psi or higher is when the bead refuses to seat properly at a lower pressure - the air is let out again as soon as the bead pops into place, and pressure adjusted to the required amount.

I would advise board members to get a proper pressure gauge if you are not at all comfortable with a tyre repair/replacement, then you can show the fitter the error of their ways. I can recommend those manufactured by PCL. Keep it in the car and it can be used when you add air at the petrol station - they are particularly prone to being way out on their figures (I found a Tesco one a few years ago that was 5psi off!)

MM

[9873210]

My car has tyre pressure sensors that work in bars not psi. My tyre pressure when the car has been stood for a while has been very steady all summer at 2.3 bar. Now the mornings are cold the readings are 2.0 or 2.1. When I've been driving for a while the pressures increase to 2.2 or 2.3

I've got an electric tyre pump which shows the tyre pressure when cold at 2.2 or 2.3. When I drive to the local filling station the tyres have warmed up and are at pressure.

It's an irritating problem because the low tyre pressure warning takes out half of the info on the speedometer display and I have to reset it back to normal display every morning or everytime the car tyres cool down. I am considering increasing the tyre pressure slightly to compensate for the cold mornings but I'm not sure if that will mean I'm driving round with overpressured tyres most of the time.

What's the tyre pressure when you've been driving around for a while in the summer? I'd guess it's 2.5 of 2.6 bar. Engineers know that tyre pressure increases when warm and allow for that. If you inflate the cold tyres to spec you will not be driving on over inflated tyres when warm, unless you do something extreme, like Narvik to Timbuctoo.

I'd be a little worried if the tyre pressure warning is going off. Typical these are calibrated to warn for significant underinflation. IMHO they are not warning you a tyre is underinflated, they are warning you that you are not maintaining your tyres properly and need to change your ways.