Tyre Pressure: The MOT Warning You Can't Ignore

The MOT's Stance on Tyre Pressure

The MOT inspection specifically checks for tyres that are significantly underinflated. The reasoning behind this is multifaceted and centres on safety and vehicle performance. A tyre that is not inflated to the manufacturer's recommended pressure is considered a safety deficiency. The MOT testers are looking for tyres that are visibly deflated or are so low on pressure that they could compromise the vehicle's handling and braking capabilities. It's not about a few PSI below optimal; it's about a level of underinflation that poses a tangible risk.

Regulation 30 of the Road Vehicles (Construction and Use) Regulations 1986, and its subsequent amendments, form the basis for many MOT requirements. While the MOT manual doesn't specify an exact PSI threshold for failure, it mandates that tyres must be maintained in 'a condition that does not cause a danger to any person'. Critically, it also states that 'tyres must not be fitted with a bead seal that has been temporarily repaired'. This implies a focus on the integrity and proper functioning of the tyre, which is directly impacted by its inflation level.

Why is Low Tyre Pressure So Dangerous?

The consequences of driving on underinflated tyres extend far beyond a potential MOT failure. Here are the primary reasons why maintaining correct tyre pressure is paramount:

• Compromised Handling and Steering: When tyres are underinflated, their sidewalls become softer and more flexible. This leads to a sluggish and imprecise steering response. The vehicle may feel 'mushy' or unresponsive, making it harder to control, especially during cornering or emergency manoeuvres. You might find yourself oversteering or struggling to keep the car in a straight line.
• Increased Braking Distances: Underinflated tyres do not make proper contact with the road surface. The contact patch, the area of the tyre that touches the ground, is distorted. This reduced and uneven contact means that your brakes cannot work as effectively. Consequently, your stopping distances will be significantly longer, putting you and other road users at risk.
• Uneven Tyre Wear: Underinflation causes the outer edges of the tyre tread to wear down faster than the centre. This premature and uneven wear not only shortens the lifespan of your tyres but also reduces their grip, especially in wet conditions. It can lead to a situation where the centre of the tyre is still perfectly legal, but the edges have worn down to the legal limit, necessitating replacement.
• Reduced Fuel Efficiency: When tyres are underinflated, they create more rolling resistance. This means your engine has to work harder to move the vehicle, leading to increased fuel consumption. While the exact percentage varies depending on how underinflated the tyres are, it can be a noticeable difference over time, costing you more at the pumps.
• Overheating and Blowouts: This is arguably the most severe risk. Underinflated tyres flex excessively, especially at higher speeds. This constant flexing generates heat within the tyre's structure. If the heat builds up to a critical point, it can cause the tyre's internal components to break down, leading to a sudden and catastrophic tyre blowout. A blowout can cause a driver to lose control of the vehicle instantly, with potentially devastating consequences.

How to Check and Maintain Tyre Pressure

Fortunately, maintaining correct tyre pressure is a straightforward process that every driver can easily perform. Here's how:

1. Find Your Vehicle's Recommended Tyre Pressures

The recommended tyre pressures for your specific vehicle are not found on the tyres themselves. Instead, they are usually located in one of the following places:

• Inside the driver's side door frame or pillar.
• In the glove compartment.
• On the fuel filler flap or cap.
• In your car's owner's manual.

You'll typically find two sets of figures: one for normal load conditions and another for a fully loaded vehicle. It's important to use the correct pressures for your driving situation. These figures will be given in PSI (pounds per square inch) and/or BAR.

2. Invest in a Reliable Tyre Pressure Gauge

While many garages offer free air, it's a good idea to have your own reliable tyre pressure gauge. Digital gauges are generally more accurate and easy to read. You can purchase these from most auto parts stores or online retailers.

3. Check Your Tyre Pressure Regularly

The best practice is to check your tyre pressure at least once a month, and also before any long journeys. It's crucial to check the pressure when the tyres are cold. This means the vehicle hasn't been driven for at least three hours, or has only been driven for a very short distance at low speed (e.g., to a nearby petrol station). Driving even a short distance can increase tyre pressure by several PSI due to friction and heat build-up.

4. How to Check and Adjust

• Remove the valve cap from one of your tyres.
• Firmly press the tyre pressure gauge onto the tyre's valve stem.
• Read the pressure displayed on the gauge.
• Compare this reading to the recommended pressure for your vehicle.
• If the pressure is too low, use an air compressor (at a petrol station or your own) to add air in short bursts, re-checking the pressure frequently until you reach the recommended level.
• If the pressure is too high, press the small button on the gauge (or use the pin on the back of some gauges) to release air until the correct pressure is achieved.
• Replace the valve cap securely.
• Repeat the process for all four tyres, and don't forget the spare tyre if your vehicle has one!

Tyre Pressure vs. Tread Depth

It's important to distinguish between tyre pressure and tread depth, both of which are MOT checks. While tread depth is checked for wear down to the legal limit (1.6mm across the central three-quarters of the tread width), tyre pressure is assessed for its impact on safety and vehicle operation. A tyre with adequate tread depth can still fail its MOT if it's significantly underinflated, due to the safety implications discussed.

Common Misconceptions

• 'You should overinflate tyres for better fuel economy': While slightly overinflating might marginally improve fuel economy, it can lead to uneven wear on the centre of the tyre, reduced grip, and a harsher ride. It's best to stick to the manufacturer's recommendations.
• 'Tyre pressure doesn't matter if the tyre looks okay': Visually inspecting a tyre is not a reliable way to determine its inflation level. Tyres can appear normal even when significantly underinflated, especially if they are run-flat or have strong sidewalls.
• 'Modern tyres don't need checking': All pneumatic tyres lose pressure over time due to natural permeation. Regular checks are essential for all types of tyres.

Table: The Impact of Underinflated Tyres