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There have been a couple of questions about tyre air pressures at low temperatures: 1, 2. But what about at high temperatures?

Currently in Australia daily highs of 35C are common, spiking up to 38 or 40C on some days. The road and tyre temperature would probably be higher, being black and under friction.

Yesterday I got fuel during the middle of the day, and also filled my tyres as it had been quite some time since I had, and they felt flattish. But they were only 1PSI or less than the target. If they're at the right pressure, then does that mean all is good? Or could it mean that when I drive at night after the car's been in the garage all day, that the pressures could drop significantly?

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Vehicle manufacturers quote recommended pressures with the tire cold (meaning: not immediately after driving) and at "typical" atmospheric conditions. Essentially, they're quoting the pressure at room temperature (20 degrees C) and typical humidity.

Air pressure will vary naturally with temperature. The relationship is roughly 5% per 10 degrees C. If you set the pressure correctly (say, 30 PSI) at 20 C with the tire cold, and then read it again when the tire is at 30 C, the pressure will read 5% higher (about 31.5 PSI in this example). That doesn't mean you need to let 1.5 PSI out of the tire! The higher reading is correct for the higher temperature. Once the tire cools back down to 20 C, it will once again read 30 PSI. The air pressure isn't really "gained" or "lost" in any permanent sense, it varies directly as the temperature changes.

So - as an answer to your question - if you know you're reading the pressure at a much higher temperature, you should fill the tire to a correspondingly higher pressure. For your example, if you're taking the reading with the tire at 40 C, you should over-fill by about 10%.

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    This answer focuses on the "correctness" of tire pressure according to the owner's manual. However, there is of course a different type of correctness of tire pressure and that is according to physics and driving experience. In the latter case the actual pressure matters very much for the handling of the car. Thus at some point it might indeed be useful to run your tires at a pressure that is "incorrect" according to the above definition but beneficial for your driving experience. It'd be great if an expert could weigh in on this. – user2705196 Dec 10 '19 at 18:13
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    The "experts" agree with this answer. The ETRTO (European Tire and Rim Technology Organization) states in their ETRTO 2001 standard that hot pressures can be as much as 20% above cold pressures, and that under no circumstances should you decrease pressure during warm weather to try to account for this - since doing so can leave the tire dangerously under-inflated once it cools down. Of course, there's always a degree of subjectivity in terms of real-world practicality, but I think there's a strong argument for simply inflating according to guidelines unless you have a strong reason not to. – dwizum Dec 10 '19 at 19:11
  • I can't find the ETRTO standard online, but Bridgestone suggests filling hot tires higher, as do Tire Rack. Generally, it's considered worse to have under-inflated tires when cold, than over-inflated when warm, so industry experts err on the side of over-inflating. – dwizum Dec 10 '19 at 19:13
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A rough calculation is that tires lose 1 psi per 5°C, this varies depending on the volume of the air in the tire, it could be less or more than this. So if you have a hot tire on a hot day which is 1psi under ideal pressure and the temperature drops 20°C your tire pressure will drop maybe another 4 psi.

So if your tires are under-inflated at the hottest part of the day then they are going to be much more under-inflated when it cools down, and you should add some air. I'd inflate to the right pressure when the tires are warm, not at the coldest part of the day or hottest, that way you split the difference.

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The proper way to maintain proper tire pressure is to check and adjust pressure after vehicle has been setting for at least 3 hours. When driving the car, the pressure will rise a bit from heat caused by friction, but that's normal. In the winter, because of colder temperatures, the pressure may get a little lower, so simply adjust to proper inflation. Pressure should be kept at manufacturers specification which is usually found on a placard on the driver's side door frame.

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