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At least in the United States, most people don't pump cold tires. They drive to the gas station and immediate pump. Yes, mechanics pump cold tires, but mechanics know, or have resources to look up, how much to pump; owners don't as easily. Why, then, do labels on cars list only the pressure to which to pump cold tires, and not the pressure to which to pump warm ones?

  • Welcome to Motor Vehicle Maintenance & Repair! Really, if you drive directly to the gas station (and it's not miles and miles away), your tires haven't built up enough heat in them to worry about. – Pᴀᴜʟsᴛᴇʀ2 Apr 30 '18 at 20:40
  • It would be possible to produce a tyre pressure chart for "warm" tyres. The problem is, how warm. Cold tyres are tyres at ambient temperature. Warm could be any number of temperature points. Plus, how do you measure tyre temperature and how do you account for the effect that adding air at a different temperature has on the overall tyre temperature. – Steve Matthews May 1 '18 at 10:14
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Because "cold" pressures are much more likely to be consistent then "warm" pressures and barring things like one side of the car having been in hot sun for a few hours should be pretty evenly "cold" all round as well.

Driving a mile or two to the garage to check pressures and inflate them is unlikely to have a significant impact on tire temperature so is generally considered "close enough", particularly for road cars which aren't going to be hugely sensitive to small variations in pressure anyway.

On the other hand if you have done a substantial drive to get there the factors affecting temperature/pressure will vary hugely, how much acceleration have you done? How much hard braking? (Heat dissipated from the front brakes can heat tires significantly - and usually much more so than rears) Have you cornered hard? Have you done hard cornering in one direction but not the other? Has one wheel gone through a deep puddle of cold water?

All of these things are big unknowns that make trying to pin down a figure for what a normal "warm" pressure is very difficult hence these are given as "cold" pressures from which the manufacturers can extrapolate that under "normal" usage the tires will be within their operating range.

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My guess is that cold pressure is a more repeatable measure. Warm or Hot tire pressure can be skewed by the amount of moisture in the air inside the tire. If you take two tires, one with air with 75% relative humidity and one with 0% RH, then heat them up. The tire with moisture will generate higher pressure. The air in the two tires have a different coefficient of expansion.

And, if you measure your tires cold, the working pressure is going to be above the cold reading. If you measure your tires hot then working pressure is less easy to guesstimate and will likely be lower than the hot reading. Under-inflated tires are bad!

Also, one of the reasons for the change to Nitrogen in auto tires is that Nitrogen is less hydroscopic, i.e. it has less capacity to absorb water. And presumably will be less affected by the reading temperature.

  • The word is hygroscopic... – Solar Mike Apr 30 '18 at 21:49
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    I'm confused. I seem to recall one of the gas laws that states that at a given pressure and temperature, there will always be the same number of molecules in a given unit of volume regardless of the gaseous element/compound. This means if the "air" is H20, CO2, O2, N2, etc. the same volume of air at the same temperature will have the same number of molecules, so I don't see how humidity could affect tire pressure as long as the temperature is above the dew point in the tire. – BillDOe Apr 30 '18 at 23:04
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    This is really not more than a guess. Humidiy does not affect the pressure unless it condensates in the cold tire. And most arguments of N2 in tires are scam. Air is not hygroscopic, and itself consists of 80% N2. That 20% O2 don't change much. The only difference concerning humidity is that N2 from the bottle is extremely dry. – sweber May 1 '18 at 17:38

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