Reference temp for recommended tyre pressure

[Swbimmer]

I am trying to follow the tyre pressure recommended by BMW (which is 2.4 bar). I also know tyres need to be cold. But at what temperature? A month ago my tyres were 18-20C in the morning before being driven and I had them at 2.4 bar. Nowadays they are about 12C in the morning given the change in weather. They have dropped to 2.2 bar.

I can't seem to find the reference temperature at which the tyres need to be at the 2.4 bar. Any advice?

[giveitfish]

Lotus manuals are pretty specific on this, basically check at ambient temperature when tyre is cold - would same apply for you?

http://www.sandsmuseum.com/cars/elise/information/...

[vikingaero]

They should be set at 2.4bar at the lowest temperature. If you inflate to 2.4bar at 20C and they drop to 2.2bar at 12C then in theory they are underinflated. At 2.4bar at 12C they will increase in pressure when the ambient temp increases, but they will be "correct" when cold.

I read somewhere a long time ago that the correct tyre pressure should be whatever the manufacturer recommends at 0C, even if that meant taking the tyres off a car and placing them in a freezer set for 0C (Setright maybe?)

[swisstoni]

I vaguely remember the idea of Standard Temperature and Pressure from my O Level chemistry days.

The temperature more or less equated to ‘room temperature’ or 20c.

[Pica-Pica]

You may be over-thinking this. ‘Cold’ generally means not driven for several hours. So set them at that condition, in the prevailing ambient temperature. Then adjust up for comfort and wear pattern as you see fit. I set mine nearer to the 5 person setting than the 3 person setting. For staggered tyres, the rear recommended pressures will a closer difference between 3 and 5 people loading than would be the case for non-staggered tyres. Of course, you may wish to adjust the temperatures in winter time. I have found that my three gauges (TireTek, Halfords, Draper) all read the same and show about 2 psi more than the TPMS does at drive away, the TPMS then rises to about 1 psi above that hand-gauge measure. (I switch between psi and bar for pressure units occasionally). I find run-flats hold pressure well.

[Green1man]

I’m THINK my car stipulates 20C as being the default ‘cold’ temp. Hence if checking at 10C or 0C say you would expect them to be say 0.1 bar or 0.2 bar lower in the colder temp. I disagree with the above comments. If say 30psi at 20C cold is correct then 30psi at 0C is NOT correct. When the car is running, say on a motorway the tyre will warm up to a similar operating temperature whether the ambient temp is 0C or 20C, so if you top up to 30psi at 0C you are going to be overinflated once the tyre warms up.

Of course this doesn’t really answer your question. I.e. if I set them to 30psi at 20C what should I set them to at 10C or 0C. Usually with mine I just check they are all about the same. If I have set them correctly in Summer then on a cold October morning they are likely all 3-4 psi lower.

[Swbimmer]

Similarly I have also seen 20C being listed as reference but this was neither from an auto manufacturer nor from tyre manufacturer so I did question the accuracy.

[Pica-Pica]

Green1man said:

I’m THINK my car stipulates 20C as being the default ‘cold’ temp. Hence if checking at 10C or 0C say you would expect them to be say 0.1 bar or 0.2 bar lower in the colder temp. I disagree with the above comments. If say 30psi at 20C cold is correct then 30psi at 0C is NOT correct. When the car is running, say on a motorway the tyre will warm up to a similar operating temperature whether the ambient temp is 0C or 20C, so if you top up to 30psi at 0C you are going to be overinflated once the tyre warms up.

Of course this doesn’t really answer your question. I.e. if I set them to 30psi at 20C what should I set them to at 10C or 0C. Usually with mine I just check they are all about the same. If I have set them correctly in Summer then on a cold October morning they are likely all 3-4 psi lower.

The handbook only refers to ‘cold’ as being ‘when not having been driven for more than 2km, or 1.25 miles, or after having been standing for more than two hours’. Set them under those conditions, tyre size and type (as per door label) and passenger/baggage loading. Then adjust to personal preferred comfort setting, but never below the tyre label setting.

[f1nn]

Road car, driven on the road?

It’ll make sweet FA difference if you set your tyre pressures at 15 or 20 degrees ambient temperature.

[Ron99]

f1nn said:

Road car, driven on the road?

It’ll make sweet FA difference if you set your tyre pressures at 15 or 20 degrees ambient temperature.

I'd guess probably about 0.5 - 0.75 PSI difference. So, yes, basically an undetectable difference.

I would also guess about 2.5 - 3 PSI difference between freezing and 21'C.

[Swbimmer]

f1nn said:

Road car, driven on the road?

It’ll make sweet FA difference if you set your tyre pressures at 15 or 20 degrees ambient temperature.

Yes normal road car

[irc]

It's a long time since I sat Higher Physics but I think pressure is directly proportional to temp. Absolute temp in Kelvin. 12C is 275K increase to 20C, an 8C increase gets to 283K around a 3% increase in pressure. Or around 1PSI for most cars.

Probably within the error of most pressure gauges. In other words not really worth worrying about.

As I said sat my physics back in the 70s so happy to be corrected.

[wong]

irc said:

It's a long time since I sat Higher Physics but I think pressure is directly proportional to temp. Absolute temp in Kelvin. 12C is 275K increase to 20C, an 8C increase gets to 283K around a 3% increase in pressure. Or around 1PSI for most cars.

Probably within the error of most pressure gauges. In other words not really worth worrying about.

As I said sat my physics back in the 70s so happy to be corrected.

12 degrees centigrade would be 285(.15) degrees Kelvin.
20 degrees centigrade would be 293(.15) degrees Kelvin.
But yeah, insignificant on road cars. Probably get similar variation between all 4 tyres.

[ashenfie]

Correct me if I am wrong but the cold temperature is outside 20c. So if it's cold weather and the stated pressure is 2 bar and the temp. is 10c then pump up to 1.9 bar.

[james_gt3rs]

I thought they said to set cold, so that when you drive and warm up the tyres a bit, it's running ideal pressure. So the ambient temp is irrelevant? E.g start at 30psi, and they rise to ~33psi when you've been driving a bit..

[Swbimmer]

Ambient, cold and warm I suppose are all relative. Tyres can reach 36C after some (motorway) driving. Cold can mean 23c in the summer and close to zero in the winter... Thus my question on what manufacturers use when they set the recommendation that shows up in your doorjamb

[Debaser]

As has been stated, set your pressures when the tyres are cold (when you haven’t driven for several hours). If they are at ambient temperature you’ll be fine. Manufacturers don’t expect you to put your wheels in the freezer before setting pressures, they expect you to set them at ambient.

[Debaser]

Swbimmer said:

Ambient, cold and warm I suppose are all relative. Tyres can reach 36C after some (motorway) driving. Cold can mean 23c in the summer and close to zero in the winter... Thus my question on what manufacturers use when they set the recommendation that shows up in your doorjamb

In this example they expect you to set the pressures with the tyres at 23c in summer, and zero in winter.

[Krikkit]

giveitfish said:

Lotus manuals are pretty specific on this, basically check at ambient temperature when tyre is cold - would same apply for you?

http://www.sandsmuseum.com/cars/elise/information/...

Exactly this - cold = ambient, which means the pressures are roughly right for the time of year.

Can't easily check your tyres at 20° in the middle of a Scottish winter, or 5° in summer in Southern Spain.

[Swbimmer]

so you are basically saying as we go towards winter and temperatures keep dropping (and thus pressure drops), we need to keep topping up the pressure every so often?