Desk sheet - Tires
Why the Pressure Reads Differently When You Get Back, and What It Means
The most misread thing on a car after a long stay. Pressure changes with temperature, so the number you see on return is telling you about the weather as much as about the tire, and separating the two is easy once you know the rule.
01The rule, first
Pressure rises when air is hot and falls when it is cold, by roughly one pound per square inch for every ten degrees Fahrenheit of change. That is a physical fact about gas in a sealed container and it applies to every vehicle in every lot.
So the reading you get on return depends on three things: what you left it at, how warm it was when you set it, and how warm it is when you check it.
02The two cases people actually meet
You left in June and came back in June. Both ends hot, so the reading should be close to where you left it. If it is well down, that is a leak rather than the weather, and it is worth looking at before you drive out.
You left in October and came back in January. That is a genuine temperature drop, and the tires will read low without anything being wrong. Perfectly normal, and the answer is air rather than alarm.
The reverse catches people too. Leave a car in February and collect it in June and the tires can read above the door sticker, because the air in them is thirty degrees warmer than it was when you set them.
| What you observe | Most likely cause | What to do |
|---|---|---|
| All four down by a similar small amount, and it is colder than when you left | Temperature | Add air to the door sticker figure |
| All four up a little, and it is hotter than when you left | Temperature | Leave it. Do not bleed pressure out of a hot tire |
| One down and three normal | A slow leak | Look at it before driving out of the lot |
| One flat | A leak or a puncture | Deal with it in the lot rather than on the road |
| All four very low regardless of temperature | Long standing plus small losses across all four | Add air, then watch them over the next week |
03The other thing that happens to a tire that stands
A vehicle left in one position for several weeks can develop a temporary flat spot where the tire meets the ground. It feels like a slight vibration for the first few miles and then goes away as the tire warms and returns to shape.
That is normal, it is not damage, and it is more noticeable after a month than after a week. If a vibration does not go away after ten minutes of driving, it is something else and it is worth stopping for.
04Why nobody here touches it
Because it is one of the five refusals, and pressure is the single most requested exception.
Nobody at any lot we place a reservation with adjusts pressure, and this desk does not arrange it. The reason is not laziness: setting a pressure on a hot tire in a lot at four in the afternoon produces a number that is wrong by morning, and doing it wrong on somebody else's vehicle is worse than not doing it at all.
What you get instead is this page, in advance, so you know what you are looking at when you get back.
05The one piece of preparation worth doing
Set the pressures to the door sticker figure before you leave, and note the date. That gives you a known starting point, which turns an ambiguous reading on return into a simple subtraction.
Nothing else. Do not over inflate on purpose to allow for loss over a month; you are guessing at two variables instead of one.
06What this means for the reservation you should make
Nothing about pressure changes which band or grade you should book. This page exists so that a long stay does not end with an unnecessary worry in a parking lot.
The one connection: the longer the stay, the more likely a temperature swing between the two ends of it. If your run crosses a season boundary, expect a reading that is not the reading you left, and check the calendar for which way it will go.