Cars – like the rest of our lives ‑ now have so many electronic systems it’s easy to trust them for everything. But how accurate are Tyre Pressure Monitoring Systems (TPMS) and our reader asks can I rely on my Tyre Pressure Monitoring System to tell me when to pump up my tyres?

What is a Tyre Pressure Monitoring System?

New cars from the end of 2014 onwards must be fitted with a Tyre Pressure Monitoring System by law. This is something that measures the air pressure of the tyres and alerts the driver when they don’t have sufficient air in them.

How do they work?

There are two types of TPMS: direct and indirect. They work totally differently and one is more accurate than the other.

How does indirect TPMS work?

Cars are packed with sensors these days. Among those are ABS (Anti-lock Braking System) sensors mounted on each wheel. These monitor wheel speed so that the ABS can operate to stop the wheels locking when you brake heavily.

These sensors can be applied to gauge tyre pressure because the less air a tyre has in it, the smaller radius it has. This means it will have to spin faster to keep up with the other three tyres. An over-inflated tyre will spin slower than three normally inflated tyres.

This information will be relayed back to the car’s computer which then tells the driver than one particular tyre is underinflated.

The problem with indirect TPMS

The trouble with the older indirect systems is that they’re not particularly accurate. Newer systems use more than one reading. They employ sensors from the vehicle stability assist system and in addition to wheel speed, they monitor yaw, g forces, brake pressure, steering angle and outside pressure.

If it detects that present pressures have dropped below a 25% threshold, it alerts the driver to which tyre is at fault. The trouble is, 25% is a lot of pressure. And if you’re relying on your tyres to be only running at three quarters pressure before you put air in them, they will have less than the ideal amount of air in them for a considerable amount of time.

How does direct TPMS work?

These systems have individual sensors within the tyre, mounted to the wheel rims and frequently part of the valve stem. The sensors send out a signal to the TPMS module telling it how much air pressure the tyres have in them.

The problem with direct TPMS

These systems are less popular with car makers because they are more expensive. They require special sensors rather than using those that are already present. And the sensors can suffer from battery failure, which means they don’t work and the battery/sensor must be replaced. Most have a life expectancy of five to 10 years.

Our advice?

In this instance, we just don’t think TPMS is accurate enough to rely on. We would advise you to check your tyres every couple of weeks in the traditional way. It only takes a few minutes and it could save you money in fuel as well as helping you to get more miles out of your tyres.