I have noticed that some cars come with a tyre-pressure monitor that displays the pressure in each tyre. My car does not have one and neither did any of my previous cars. How does this work and is it possible to retrofit such a system to my car so that I can be warned when a tyre is deflating?

The tyre pressure-monitoring system is increasingly common in new cars.

Usually, the system uses wheel speed sensors in the anti-lock braking system to compare rotational speed of the tyres. Hence when one tyre begins to deflate, its change in diameter causes an increase in rotational speed when travelling in a straight line. A warning signal will appear in the instrument cluster and identify the tyre which has experienced loss of pressure.

More sophisticated systems use individual sensors so that the exact pressure of each tyre is displayed. More than just deflation or impending flat tyre, such systems provide information on whether the pressures are correct and if they are the same on each wheel.

Although it sounds rather elaborate, this is the type of system that can be easily retrofitted to any vehicle.

Pressure sensors fitted directly onto the tyre valve look like valve caps. Each sensor has its own built-in battery and is specifically coded to identify the location of the tyre. A radio-frequency signal is sent to a mini processor inside the car that displays the signals as individual pressure readings.

If you are afraid the sensors might be stolen, you can opt for internally mounted sensors. However, this requires the services of a tyre shop since you will need the tyre off the rim for installation.

This is theft-proof but not so easy to replace if a sensor fails.

In either model, the sensors would need periodic replacement as the batteries are not rechargeable. The sensors are inexpensive, with a complete set costing less than $60 at online stores.

Correct tyre pressure is essential for optimum all-round performance. Motorists often pay little attention to tyre pressure. Under-inflation can affect handling and fuel consumption adversely, while over-inflation undermines comfort. Both under-and over-inflation cause uneven tyre wear.