You've got the route planned, the bags are by the door and the car has had a wash. What it probably hasn't had is five minutes of attention at each corner. Tyres are the only part of the car that touches the road, and the two things that decide whether they do that job well — tread depth and pressure — are also the two things most drivers check least often. Neither job needs a ramp or a workshop, and the kit costs less than a tank of fuel. Here's how to do both properly before you point the car at a motorway and settle in.

What the law actually requires

For cars and light vans in the UK, the legal minimum tread depth is 1.6mm. It applies across the central three-quarters of the tread width, all the way around the tyre — not just the strip you can see when you crouch down at the kerb. That detail catches people out. A tyre showing 3mm on the outer edge and 1.4mm on the inner shoulder is illegal, and the penalty is charged per tyre, so a set of four can turn into a very expensive morning.

Most tyres have wear indicators: raised bars set into the main grooves at 1.6mm. When the tread surface sits level with those bars, the tyre is at the limit and, in legal terms, finished. Think of them as confirmation that you have already left it too late.

And 1.6mm is a floor, not a target. Tread is what clears water from between rubber and road. The less of it you have, the longer a wet stop becomes and the sooner a tyre will lose grip entirely. Many drivers replace at around 3mm, particularly if they cover motorway miles or drive a lot in winter. If you are unsure how the rules apply to your vehicle — a van, a trailer, a motorhome, or a car registered elsewhere — check with a tyre specialist or your licensing authority.

Find the right pressure for your car

The correct pressure for your car is not printed on the tyre sidewall. That number is the maximum the tyre can hold at full load, and it is almost always higher than what your vehicle actually needs. The real figures live on a placard, usually in one of these places:

• the driver's door shut, or the door jamb itself
• inside the fuel filler flap
• inside the glovebox lid
• in the vehicle handbook

You will often see two sets of numbers: one for normal running, one for a fully laden car. If you are heading away with four passengers and a packed boot, use the higher figure. Some cars also list a separate pressure for the rear tyres when towing. Ignore the second set and a heavily loaded car will feel vague and floaty at speed, and the rear tyres will run hot.

Measuring tread depth with a gauge

A mechanical pencil-type tread gauge costs very little and never runs out of battery; digital versions are easier to read and only slightly more expensive. Either will do the job. So will the gauge on most decent pressure checkers.

1. Park on flat ground, switch the engine off and apply the handbrake. Work in daylight, or with a torch.
2. Find the main grooves — the wide channels running around the circumference of the tyre, not the narrow slits cut across the blocks.
3. Push the gauge's probe into a groove and press the flange flat against the tread blocks on either side. Keep it straight; a gauge held at an angle reads deeper than the tyre really is.
4. Read the measurement. On a mechanical gauge the barrel slides out and you read where it stops. Digital ones settle in a second or two.
5. Repeat at three points across the tyre — outer shoulder, centre and inner shoulder — and at two or three positions around the circumference. Note the lowest figure you find. That is your tyre's number.
6. Do all four. Front tyres wearing faster than rears is normal on a front-drive car. One edge of a single tyre wearing faster than the other is not, and usually points to alignment or a suspension problem.

The old trick of dropping a 20p coin into a groove gives you a rough idea, but coins wear and grooves vary, so treat it as a nudge to go and find the gauge rather than a measurement.

Checking pressure without fooling yourself

Pressure should be checked cold — before you drive, or after the car has stood for a few hours. Tyres warm up as you drive and the air inside expands, so a reading taken at a motorway services will be noticeably higher than the true cold figure. That matters, because the mistake people make there is letting air out to "correct" a hot tyre, which leaves them under-inflated and running hot for the rest of the trip.

The routine is simple: unscrew the valve cap, press the gauge on square, and accept the brief hiss as the gauge seals. Read the number, write it down, and compare it with the placard. Top up at a garage if you need to, then recheck. Refit the caps — they keep grit and water out of the valve. Where you can, use your own gauge; forecourt ones take a lot of abuse and are not always accurate.

Two things people forget. First, the spare: a space saver often runs at a much higher pressure than the road tyres, so check its sidewall or the placard. A spare sitting at 20psi is no help at the roadside. Second, a tyre that needs topping up every week or so has a puncture, a leaking valve or a corroded rim — get it looked at rather than living with the pump.

When a tyre has to be replaced

Pressure and tread are the routine checks, but a few other things put a tyre beyond saving:

• At or near the limit. Tread at 1.6mm, or level with the wear indicators, means replacement now. Approaching 3mm is the sensible point to start shopping.
• Sidewall damage. Cuts, cracks, or a bulge in the sidewall. A bulge means the internal structure has failed — that tyre is not repairable.
• Punctures in the wrong place. A nail in the central tread can often be repaired to a proper standard. Damage to the sidewall or shoulder generally cannot. Let a specialist assess it.
• Uneven wear. A worn inner or outer shoulder, or a band worn down the centre, tells you something about pressure or geometry. Fitting a new tyre without fixing the cause simply wears the new one the same way.
• Age. Rubber hardens whether or not you use it. The four-digit code on the sidewall gives the week and year of manufacture. Many manufacturers suggest having tyres inspected from around five years old and replacing them at ten regardless of tread. If you don't know a tyre's history, have it looked at.
• Vibration or pulling. It may be the tyre, the wheel or the balancing. Diagnose it before you buy rubber.

The night before, not the morning of

Do the checks the evening before you travel. The light is better, the garages are still open, and if you find a slow leak or a tyre that has quietly gone past its best, you have time to do something about it. Keep a pressure gauge and a tread gauge in the glovebox; both together take up less space than a packet of wipes.

Then, on the morning, a two-minute walk around the car: look at each tyre, feel the sidewalls for bulges, make sure the spare is present and inflated, and check nothing is trapped in the tread. When you stop for fuel halfway, look at the tyres again — but leave

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