Is your pressure wrong, or is it just the temperature?
Tell it what you normally run and what the gauge says now. It takes today's temperature for your town and answers in three words, then shows when to check and what your tyres will read across the day.
Air pressure moves with temperature, so the same air in the same tyre reads differently morning and afternoon. Roughly 1 PSI for every 6 degrees. If you normally run 32 and the gauge says 34 on a warm afternoon, nothing has gone wrong: that is your 32, warmer. Let it out and you will be under on the next cold morning.
- The tool needs two numbers: what you normally run, and what the gauge says now. It takes today's temperature from your town and works out the rest.
- Check them cold, before you drive. A tyre warmed by driving reads 3 to 6 PSI high, and that reading is not the one to judge by.
- Never let air out of hot tyres to correct a high reading. You will be badly under-inflated once they cool.
- Not sure what you should be running? That is a different question, answered by the tyre pressure calculator.
In this guide
How toHow to use it
Nothing here is a setting you have to get right. Two numbers and a town, and the tool does the rest. It is checking one thing: whether the difference between those two numbers is explained by how warm or cold it is today.
- Enter what you normally run The figure you set them to, not what you think they should be. If you do not know it, the tyre pressure calculator works it out from your vehicle, and your placard beats both.
- Enter what the gauge says now Whatever you have just read, warm tyres included. That is the point: the tool is here to tell you whether that reading is a problem.
- Pick your town It pulls today's real temperature for that town, and today's low, which is the one that matters. No town, or the weather is unavailable? Use set it myself and type the temperature.
- Read the verdict, and the workings under it You get one of three answers, and underneath, the numbers it used so you can see why.
| What it says | What it means | What to do |
|---|---|---|
| Nothing to do | The difference is the temperature, or too small to chase. | Leave them alone. |
| Reads high, check it cold | Even at today's coldest the air still reads above your figure, so the weather does not explain all of it. Warm tyres do the same thing. | Check again cold, before you drive. Do not let air out of a warm tyre. |
| Add about n PSI | They are genuinely low, beyond what the cold accounts for. | Top up while they are cold. The chips underneath show what being low is costing you. |
The shots below are shrunk to fit. Click any of them to enlarge, then use the arrows to move through the set.
The physicsWhy the number changes on its own
The air sealed in your tyre has nowhere to go, so when it warms it pushes harder on the walls and the gauge reads higher. Cool it and the opposite happens. Nothing enters or leaves the tyre.
The relationship is pressure against absolute temperature, which for a tyre around 32 PSI works out at about 1 PSI per 6°C. A 15 degree swing between a cold dawn and a warm afternoon is therefore about two and a half PSI on the gauge, from exactly the same air. That is why two honest readings taken hours apart can disagree, and why the fix is almost never to add or release air in the middle of a hot day.
How to checkCold means before you drive
Tyre placards and every pressure recommendation are written for cold tyres, and cold has a specific meaning: the car has been standing for at least three hours, or has covered less than a couple of kilometres. Driving heats the air by friction and flexing on top of whatever the weather is doing.
- Best: first thing in the morning, at home, before the car moves.
- Acceptable: any time the car has sat for a few hours in the shade.
- Worst: at the service station after a drive, which is exactly when most people check, and why so many tyres get let down when they were fine.
The 24 hour strip in the tool shows this for your own town: the coolest hours are the honest ones, and it tells you what your tyres will read at each point of the day.
What mattersUnder-inflation is the one that hurts
A couple of PSI over is mostly harmless: a slightly firmer ride, marginally less grip on a rough surface. A couple of PSI under is the one that costs you. The tyre flexes more, runs hotter, wears its shoulders, uses more fuel, and takes longer to stop in the wet.
That is why the tool only shows the cost of being low. If your reading is simply the afternoon being warm, it says so and tells you to leave it alone, because chasing the daily swing is how people end up under-inflated every cold morning.
Pressure is also one of the inputs in our braking simulator, so you can see what a few PSI does to a wet stop from 100 km/h with your actual tyres.
QuestionsCommon questions
My gauge reads higher than what I set. Should I let air out?
How much does pressure change with temperature?
Why is my pressure light on after a cold night?
What about heat from driving?
What pressure should I be running?
Pressure is free. Tread is not.
Correct pressure gets the most out of the tyres you have. When the tread runs out we fit new ones in Te Puke, and we will tell you honestly what grips in your size.
(07) 573 9090 · Te Puke, Bay of Plenty
General guidance based on published standards and our own workshop data — reliable but hand-compiled and not guaranteed error-free. Spot something wrong? Email email@tyredispatch.co.nz and we'll fix it.