Correct tyre pressure, and how to check it properly
9 min readBy Wheel Fit
The short answer Your correct pressures are on a plate in the driver's door shut or inside the fuel flap — not on the tyre sidewall. Check them cold, at least monthly, and do not forget the spare.
The number on the tyre is not your pressure
This trips people up constantly. The pressure moulded into the sidewall is the tyre's maximum, not the figure your car needs. Inflating to it will give you a harsh ride, a narrow contact patch and accelerated wear down the centre of the tread.
Most cars list two sets of figures: normal load, and fully laden. If you are running four adults and luggage to the airport, the higher set is the correct one — see tyre pressures for loads and towing.
Where to look
- Driver's door shut — the most common location.
- Inside the fuel filler flap.
- The glovebox, on some makes.
- The handbook, which also covers sizes the placard may not list.
If you have changed wheel size The placard covers the sizes the car was sold with. If you have fitted aftermarket alloys in a different size, the placard figures may not apply — ask when the tyres are fitted rather than guessing.
Check them cold
Rolling heats the air inside a tyre and raises its pressure, typically by a few PSI. Check after a run and you will read high, let air out, and end up genuinely under-inflated once things cool.
"Cold" means the car has been standing for a couple of hours, or has driven less than a mile or two. If you have to use a forecourt gauge after a longer drive, expect the reading to be a little above the true cold figure.
Temperature changes pressure on its own
Air pressure falls roughly 1 to 2 PSI for every 10°C drop in temperature. That is why the first properly cold morning of autumn sets off TPMS warnings across the country, on cars where nothing is leaking.
It is not a false alarm — the tyre really is under-inflated and does need topping up. It also means pressures set in August are genuinely low by November. The TPMS warning light covers how to read what the light is telling you.
What under-inflation costs
- Wear on both shoulders. An under-inflated tyre bows in the middle, so the edges carry the load and scrub away — uneven tyre wear.
- Heat. The extra flexing generates heat, and heat is what causes blowouts at speed.
- Fuel. More rolling resistance, more consumption.
- Longer stopping distances, especially in the wet.
Over-inflation is not the safe alternative. It wears the centre strip, reduces the contact patch and makes the car skittish over bumps.
| Under-inflated | Over-inflated | |
|---|---|---|
| Wear pattern | Both shoulders | Centre strip |
| Ride | Vague, wallowy | Harsh, skittish |
| Heat | High — the dangerous one | Low |
| Grip | Reduced, especially wet | Reduced — smaller contact patch |
| Fuel | Worse | Marginally better, not worth it |
How to actually do it
- Find the plate in the door shut and note the front and rear figures — they are often different.
- Unscrew the valve cap and press the gauge on squarely. A hiss means it is not seated.
- Adjust, then re-check.
- Replace the valve caps. They keep dirt and water out of the valve.
- Check the spare while you are there. A flat spare is worse than useless.
A decent pocket gauge costs very little and is more reliable than most forecourt machines, which take a lot of abuse. If your readings and the forecourt's disagree, trust the one you can check against a second gauge.
PSI, bar and the units problem
UK placards and forecourt machines usually use PSI. Handbooks — particularly on European cars — often quote bar, and some list kPa. They are easy to convert and easy to confuse:
- 1 bar ≈ 14.5 PSI
- 100 kPa = 1 bar
- A typical car figure of 2.2 bar is about 32 PSI
Setting 32 bar is not possible, but setting 2.2 PSI by mistake certainly is.
Do not rely on TPMS alone
Tyre pressure monitoring is a warning system, not a maintenance schedule. Most systems only alert at roughly 20–25% below the correct pressure, by which point the tyre has been running under-inflated — and doing damage — for some time.
Indirect systems cannot detect all four tyres dropping evenly at all, which is exactly what happens over a winter. Monthly manual checks still matter.
If a tyre keeps losing pressure
A tyre that needs topping up every couple of weeks has a problem: a slow puncture, a corroded rim seal, or a perished valve. It is worth finding out which before it becomes a roadside job.
Why does my tyre keep losing pressure? works through the five common causes, and whether a puncture can be repaired covers what can be fixed.
Common questions
- How often should I check?
- Monthly, and before any long journey. It takes two minutes with your own gauge.
- Should front and rear be the same?
- Often not. Many cars specify higher rear pressures, particularly when laden. Use the placard figures rather than assuming symmetry.
- Is nitrogen worth paying for?
- Ordinary air is already about 78% nitrogen. The claimed benefit is slower pressure loss and less moisture. For a road car checked monthly, the practical difference is small — and it is no substitute for checking.
- What pressure for a space-saver spare?
- Usually considerably higher than a normal tyre, and printed on the spare itself or on a sticker in the boot well — see space-saver or sealant kit.
Want them checked properly?
If you would rather have all four checked along with tread, condition and age, we can do it at your home or workplace. Call 07989 606060.
