Tyre pressure reduced due to hot weather?

[HenryT]

Has anyone notice the 'cold' tyre pressure having fallen a few PSI recently?

Mine all seem to have dropped by about 3 PSI and I'm wondering if it might possibly be a result of prolonged very hot temperatures? My car lives outside and is in full sun much of the time (one of the drawbacks of living in Manchester I guess?).

 

[Mr H]

Mine did the same and i suspect the new one will too, down to the heat i reckon as it happens quite often around here ;-)

 

[MrT3]

You would expect the pressure to rise in hot weather, due to the air inside expanding more than the rubber, not fall.

 

[Nick77]

You would expect the pressure to rise in hot weather, due to the air inside expanding more than the rubber, not fall.

Warmer air has less pressure for the same volume because of that slight expansion.

 

[MrT3]

Warmer air has less pressure for the same volume because of that slight expansion.

Yes if the air can expand when it isn't contained. But it is sealed within the tyre so it cannot expand and therefore the pressure increases.

When you start driving your tyre pressure always increase from the cold starting pressure as the friction heats up the air inside.

 

[Nick77]

I'd imagine there's a little bit of give in the tyres for a tiny amount of expansion! They are rubber after all.

 

[MrT3]

I'd imagine there's a little bit of give in the tyres for a tiny amount of expansion! They are rubber after all.

Sure, and the rubber will also expand due to the temp. increase but that isn't enough to compensate for the gas pressure increase. I believe the accepted rate of increase in the industry is around 1.5 psi per 10 deg C temperature rise.

Where you can see a fall is it you fill your tyre with hot air and then it cools down, you then have the reverse effect. If, for example, you had an under-inflated tyre in the sun where the temperature of the air inside could be 50 deg C and you pumped it up to 42 psi, then when the temperature drops overnight to 15 deg C your pressure in the morning is going to be around 37 dec C.

You should always pump your tyres up when it is cool (morning/last evening) when the sun isn't shining directly on the tyres.

 

[pgkevet]

Indeed. After several punctures I keep my tyre pressures permanently displayed and note them when they first come up and adjust with the weather. My S is at 45psi cold and usually runs at 47-48psi on motorways. I’ve been parked up charging for an hour in the sun and had one side reading 49psi and the shadow side 45psi and again both changing to 48psi once stabilised on the road. Call it paranoia but if one tyre starts to go out of sync by 1psi then I adjust once cold and monitor with a digital gauge in case the TPMS is screwy. Car and compressor live in the barn.
Ambient temperature increases will not drop tyre pressure. The only reasons I can think of are (very) slow gas diffusion, leaks or as suggested inflation with hot air.

 

[ShadowSix]

Warmer air has less pressure for the same volume because of that slight expansion.

You're thinking of density.

 

[RedMod3]

PV/T is a constant. GCSE Physics. Ideal gas law - Wikipedia

 

[HenryT]

Ambient temperature increases will not drop tyre pressure. The only reasons I can think of are (very) slow gas diffusion, leaks or as suggested inflation with hot air.

I didn't think increased ambient temperature should cause pressure to reduce and would have have actually expected to see an increase, as you would do while driving. The reason I asked in the first place is that I haven't touched the tyres since taking delivery on 23 June. Until a couple of days ago they were all 42 or 43 PSI on a pretty consistent basis. Since Friday (the hottest day here - 32 C) they have read 39 or 40.

 

[hgmichna]

I regularly reduce tire pressure in spring and pump it up again in autumn. The idea is to keep the pressure near the prescribed 2.9 bar in the Model 3.

I often check the pressure before a long drive and correct, if necessary.

On a long drive the pressure rises as the tyres get warm. That's normal and taken into account by the tyre manufacturer. After cooling down the pressure normalizes.

 

[AMP_it_up]

You need to check the tyre pressures with an accurate gauge when the car hasn’t been driven for at least 2 hours and the tyres are not in direct sunlight. The tyre pressures should then be adjusted to 42 psi or your chosen pressure. Then do a reset of the TPMS and go for a drive. The car should then remember the starting pressures as the default pressures which when fully warmed up gives the equivalent of the cold pressure.

As an example my tyres were set at 42 psi with a TUV approved gauge at an ambient temp of around 15 C. First thing in the morning the pressure readout shows 39 psi which gradually increases as the tyres warm up with driving. On hot days or after 20 miles the pressure readout will go up to 44 psi or more.

 

[NATO]

H

You need to check the tyre pressures with an accurate gauge when the car hasn’t been driven for at least 2 hours and the tyres are not in direct sunlight. The tyre pressures should then be adjusted to 42 psi or your chosen pressure. Then do a reset of the TPMS and go for a drive. The car should then remember the starting pressures as the default pressures which when fully warmed up gives the equivalent of the cold pressure.

As an example my tyres were set at 42 psi with a TUV approved gauge at an ambient temp of around 15 C. First thing in the morning the pressure readout shows 39 psi which gradually increases as the tyres warm up with driving. On hot days or after 20 miles the pressure readout will go up to 44 psi or more.

How do you reset the TPMS? I thought the sensors worked out the pressures themselves?

 

[AMP_it_up]

H

How do you reset the TPMS? I thought the sensors worked out the pressures themselves?

Service Menu - RESET TPMS sensors - follow on screen instructions

 

[Nick77]

You're thinking of density.

Density is directly proportional to pressure.