Some TPMS readings are notoriously unreliable because tyres can lose a lot of air before some sensors register it. There are two types of TPMS: the lower-cost, indirect TPMS doesn't actually monitor air pressure. Instead, it uses the car’s ABS wheel sensors. These check how fast the wheels rotate and compare speeds with each other. If a tyre is under pressure it won’t go round as quickly as other wheels, the computer will work out it’s because it’s under pressure and the warning light will come on. But it won’t detect external issues, like a nail in the tyre. The lesser-used, more accurate direct TPMS measures a tyre's actual pressure by using a wheel-mounted sensor on each tyre.

According to GreenFlag, some tyre pressure monitoring systems won’t 'see' any problem until a tyre has lost around a quarter of its air — which is a lot of pressure. A tyre deflated by that much could be quite dangerous to drive on. On a less serious note, an underinflated tyre also uses more fuel. I'd suggest getting yourself a good tyre pressure gauge and basing the pressure off that reading instead. If it consistently looks a long way off the digital reading in the car, I'd recommend checking it yourself more regularly as it clearly isn't accurate enough to be trusted. Alternatively, an aftermarket TPMS system with wheel-mounted sensors could be more accurate, but you'll need to ensure the batteries in the sensors aren't flat. We tested a Michelin TPMS system a few years ago. You can find the review here: kit.honestjohn.co.uk/reviews/review-michelin-fit2g.../