Ultimate guide to car tyre pressure - What it should be, how to check it & more

Ellie Dyer-Brown, 4 months ago

5 min read

  • Tyres
  • Maintenance
  • How to
Car tyre pressure

Where would we be without tyres? Until one goes flat, they rarely get a second thought, but the role they play is vital. They are the only part of a car that touches the road, making steering, acceleration and braking possible. Without tyres, we’d be going nowhere fast - in fact, we’d be going nowhere at all.

For all these reasons, maintaining the correct tyre pressure is essential. This guide answers all your questions about tyre inflation, including how to find out the correct pressure, how to check it and how to top it up.

Contents:

What is tyre pressure and how is it measured?

What should my tyre pressure be?

Tyre pressure for popular car models

When should tyre pressure be checked?

How to check your tyre pressure

How does a tyre pressure monitoring system work?

How often should you check your tyre pressure?

How would under-inflated tyres affect your vehicle?

Can you over-inflate tyres?

Can you drive on a flat tyre?

When would you increase tyre pressure?

What causes low tyre pressure?


In a nutshell: Tyre pressure can impact fuel economy, road grip, steering and handling. You can find the correct pressure for your car’s tyres in the driver’s handbook or printed inside the fuel cap or the sill of the driver’s door.


What is tyre pressure and how is it measured?

Tyre pressure refers to how much air has been pumped into your tyres - in other words, how inflated they are. It is measured in BAR pressure (a metric unit) or pounds per square inch (PSI - an imperial unit). 

Man checking car tyre pressure.

What is the difference between BAR and PSI?

One BAR is worth approximately 14.50 PSI. So a tyre with a pressure of two BAR equates to 29 PSI.

To explain the difference in how these units are measured, we’ll have to get a little technical. BAR measures pressure as a force applied perpendicularly on a unit area of surface, while PSI measures it as a one-pound force applied on a site measuring one square inch.

Fortunately, most tyre pressure machines, such as those you find in garage forecourts, allow you to enter the pressure in PSI or BAR, so you don’t need to worry about doing any calculations. Most car manuals provide the recommended pressure in PSI.


What should my tyre pressure be?

Every vehicle model has its own specifications for tyre pressure. Most fall between 28 and 36 PSI, and the pressure recommendation will often vary depending on whether it is a front or rear tyre and how many passengers the vehicle usually carries. As such, you should check your pressure before a long journey and when carrying a significantly heavier or lighter load. 

When it comes to towing a trailer, inflating the rear tyres by an extra four to six PSI should give the required stability.

You can find information about your car’s recommended tyre pressure in the owner’s handbook or printed on the sill of either the driver’s door or the fuel cap.


The question “what pressure should my tyres be?” doesn’t have a one size fits all answer.. 

Make and modelFront tyresRear tyres
Ford Fiesta30 PSI / 2.1 BAR26 PSI / 1.8 BAR
Ford Focus33 PSI / 2.3 BAR33 PSI / 2.3 BAR
Vauxhall Corsa31 PSI / 2.1 BAR31 PSI / 2.1 BAR
Volkswagen Golf32 PSI / 2.2 BAR32 PSI / 2.2 BAR
Vauxhall Astra32 PSI / 2.2 BAR32 PSI / 2.2 BAR

We also have the following guides which may be of use:

As you can see from the table above, sometimes the front tyre pressure differs from the rear. Usually, when this is the case, the front pressure is higher to accommodate the engine and transmission, particularly in front-wheel drive cars.

Likewise, when you plan on carrying a heavy load in your vehicle, it’s recommended that you should inflate the tyres above their normal pressure. 

Rule of thumb: Tyre pressure should never drop below 15 PSI as this could result in the tyre rolling off the rim.


When should tyre pressure be checked?

You should always check your tyre pressure when the tyres are cold. So if you’re planning a journey, you should do it beforehand; otherwise, you may get an inaccurate reading.

Alternatively, if you have just driven a long distance, you should wait at least two hours before checking the car’s tyre pressure. This is because warm air is less dense than cold air. However, if you have just driven for a few minutes to a petrol station, it’s fine to check them.


How to check your tyre pressure

Equipment

Before you start, you’ll need a gauge that measures tyre pressure in the same unit as your car’s guidelines - either PSI or BAR, as discussed above.

Checking your tyre pressure

Once you’ve found a suitable pressure gauge, follow these steps.

Step one: Locate the tyre valve and remove the dust cap

Make sure to put it in a safe place so it doesn’t get lost.

Removing a tyre valve dust cap.

Step two: Attach the gauge onto the tyre valve stem

Press it down evenly so you get an accurate reading.

Attaching a tyre pressure gauge.

Step three: Look at the number on the pressure gauge

This information will tell you whether you need to deflate or inflate the tyre.

reading a tyre gauge.

Step four: Repeat

Follow steps 1-3 on all tyres, including the spare tyre, if you have one.


How does a tyre pressure monitoring system work?

Most modern vehicles have a tyre pressure monitoring system - often abbreviated to TPMS - which reports low pressure and pressure imbalances between tyres.

TPMS display.

The TPMS has a sensor that switches on a warning light when it detects a drop in pressure of between 6-7 PSI - this is now a legal requirement. However, some manufacturers set the warning light to come on for smaller decreases in pressure.

Having a TPMS is beneficial because it lets you know whether a tyre is repeatedly leaking air due to a slow puncture; finding this out sooner rather than later could help you avoid a blowout or a completely flat tyre.

There are two primary types of TPMS: direct and indirect.

Most cars have a direct TPMS since this type is the most reliable. It uses pressure sensors within the tyre, attached to the back of the valve. These sensors detect the exact pressure of the tyre and will trigger a warning if it drops significantly. Some systems also inform you which tyre has lost pressure, which saves time.

The TPMS has an internal battery - failure of this component is the main reason the system sometimes stops working. Unfortunately, the sensor is a contained unit, so you can’t just replace the battery.

Indirect TPMSs don’t have a sensor of their own; instead, they use another one inside the wheel, usually belonging to the anti-lock braking system. The sensor tracks the tyres as they revolve and measures their radius - an underinflated tyre will have a smaller radius and will thus make irregular or more turns. Although less accurate, indirect TPMSs can still inform you when a tyre needs more air. 


How often should you check tyre pressure?

It’s a good idea to check your tyre pressure once a month. That way, you can identify potential problems - such as a slow puncture - early.

Tyres naturally lose air over time. In fact, they lose between one and three PSI each month. Eventually, this adds up, and driving on underinflated tyres reduces your car’s fuel efficiency, so regularly checking them can save you money.

If you’re planning on driving a long distance, you should also check your tyres before you set off and before you’ve made any other journeys that day.

How to put air in tyres

To inflate the tyre, you’ll need a suitable pump. You could purchase one online or use a tyre pressure station at a local garage forecourt.

The advantage of using a tyre pressure station is that you can set the machine to the required pressure, and it will automatically cut out once the tyre reaches that pressure.

If you use a regular at-home pump, add a small amount of air at a time to prevent over inflation. Connect the pump to the tyre valve and turn the inflator on. Some pumps will stop when they reach the desired PSI, while others require you to check this manually.

After you’ve inflated the tyre, don’t forget to replace the valve cap! Your tyres won’t deflate if you leave it off, but the cap does play a role in keeping dust, moisture and debris out.

Deflating your car tyre

Tyre pressure stations can also deflate your tyres, but it’s easy to do this yourself - all you’ll need is a flathead screwdriver. Use the tip to push the pin in the valve stem, which allows it to release air. Do so in small amounts until you reach the desired tyre pressure.


How would underinflated tyres affect your vehicle?

Under-inflated tyres can have many negative consequences, such as the ones listed below.

1. Tyre failure

When your tyres are underinflated, more of their surface area touches the road, which causes premature wear, cracks and overheating - all of which eventually result in total tyre failure, sometimes in the form of a blowout while driving.

2. Cracked tyres

Underinflation leads to excessive friction, often causing cracks in the tyre’s sidewall. Since these cracks can’t be repaired, you must pay to replace the tyre.

3. Heavy steering

When there isn’t enough air in your tyres, the sidewall may flatten as the rubber twists, making your steering feel heavy. Not only is this uncomfortable, but it can also be dangerous because the rubber cannot grip the road properly.

4. Uneven wear

Tyres that are underinflated can stretch and bulge, resulting in uneven wear. You’ll have to replace your tyres more often, which costs more money in the long run.

5. Poor fuel economy

Increased friction means your car has to work harder than usual, negatively impacting fuel economy. As a result, you’ll have to spend more money on fuel to drive the same distance.

6. Uncomfortable ride

One of the main jobs of tyres is to absorb shocks from the road. However, when underinflated, they are unable to do so correctly, which means you’ll notice more bumps as you drive. In turn, this will wear down your suspension more quickly, and you could end up having to pay for a repair.

Once your tyres have started to crack or bulge, they can’t be repaired - that’s why it’s crucial to catch underinflation early. Topping up your tyres with air takes no longer than five minutes and is free, whereas replacing a tyre costs between £40-70 for regular tyres and even more for high-end ones. Not to mention it’s more inconvenient and time-consuming to change a tyre!

7. You could be fined

If you get caught driving on underinflated tyres, you could receive a serious fine of up to £10,000 and 3 points on your license per tyre. 


Can you overinflate tyres?

It isn’t just under-inflation you need to worry about; over-inflation also comes with risks.

The most obvious consequence of having too much air in your tyres is that they become distorted, resulting in decreased traction and increased wear and tear. In addition to having a reduced lifespan, overinflated tyres can also cause excessive noise levels, an increased risk of aquaplaning and potential suspension and steering problems.

Fortunately, it’s easy to fix overinflated tyres. 

How to fix overinflated tyres

  • Remove the dust cap on the tyre valve.

  • Locate the small pin inside the valve.

  • Use a flat-headed screwdriver or other suitably small tool to push the pin and release air.

  • Once you have done this, check your tyre pressure and add/remove air as needed until you reach the ideal pressure.


Can you drive on a flat tyre?

You should stop driving as soon as possible when you notice you have a flat tyre. Ensure you find a safe location to pull over, then change the tyre before continuing your journey. If you can’t change it yourself, plenty of mobile mechanics can do it for you no matter where you are.

A flat tyre on a Ford car.

Continuing to drive with a flat tyre can incur serious consequences - you could be fined and receive points on your license.

You could also damage your vehicle’s rims and suspension, so you’ll be looking at a more significant repair cost than simply replacing the tyre. In fact, suspension repairs can cost hundreds of pounds.


When would you increase tyre pressure?

You may need to inflate your tyres more if you’re carrying a heavy load in your vehicle. Most vehicles have recommendations for different loads - for example when one person is in the car versus five.

Adding more air to tyres helps them support the vehicle's weight and any additional weight. However, as discussed earlier, it’s possible to inflate your tyres too much. That’s why you should always consult your owner’s handbook. 


What causes low tyre pressure?

The pressure in your tyres fluctuates naturally according to the temperatures they are exposed to; in winter, you’re likely to see a slight drop in pressure.

Other common causes of low pressure include:

  • Damaged tyre bead 

  • Valve issues

  • Puncture or slow puncture

  • Osmosis

Osmosis happens naturally - it is a slow process by which air leaves through the rubber walls of the tyre. If you notice your tyre is losing pressure quickly, it’s unlikely osmosis is to blame.

Depending on the severity of the puncture, it may be possible to carry out a temporary repair using car tyre sealant. This should not be a permanent fix.

Read this guide for more information about identifying, locating, and fixing a slow puncture.

Replacing your tyres

No matter how carefully you drive your car and monitor its tyre pressure, eventually, you’ll have to replace the tyres. On average, drivers get 20,000 miles from their front tyres and 40,000 from the rear. After five years of use, it’s best practice to have your tyres inspected annually by a professional. When it’s time to replace them, WhoCanFixMyCar is here to help. 

A mechanic selecting a tyre in a workshop.

You can find the perfect fit using our handy tool, select your tyres, choose a fitter and decide whether you want the job done at a garage or home. 


Having problems with your car tyres? WhoCanFixMyCar can help you find the right mechanic at the right price, whatever you need.

If you found this guide helpful, you might also like:

Written by Ellie

Ellie Author Pic

Ellie is WhoCanFixMyCar’s Content Writer. She has a BA in English literature from Durham University, a master’s degree in creative writing, and three years of experience writing in the automotive industry. She currently drives a Suzuki Swift.

Find Ellie on LinkedIn.