Caught Speeding? Blame Your Tyres

The boys and girls in blue have every good reason to look a tad cynical when yet another motorist tries to talk their way out of a speeding ticket; during the course of their often thankless job the average police officer is the recipient of a rich assortment of colourful but implausible excuses for fast driving. But if you’re quick – and own the face of a poker player – you may be granted the opportunity of testing a brand new excuse next time the long arm of the law catches up with you: I didn’t know I was speeding, officer. It was my tyres….

An Australian researcher has drawn the conclusion that numerous motorists are being unfairly booked because their vehicle’s speedometers are underestimating how fast they’re travelling, and one of the factors leading to instrument panel incongruity is the type and condition of the rubber that comes in contact with the road.

Measurement export Les Felix, South Australian co-ordinator of the Metrology Society of Australia, believes that tyre pressure and brand of tyre are two factors that can affect speedometer accuracy. A study Felix conducted, using 30 different brands of tyre on two cars, showed variations in speedometer reading compared with real speed of up to 3km/h, while variations in tyre pressure can have a similar effect. The result is that certain types of tyre, if not correctly inflated, could lead to speedometers reading up to 6km/h low. “You could go and get a new set of tyres and drive properly and actually be speeding,” Felix told the Australian Broadcasting Corporation.

Other factors identified by Felix as leading to inaccuracies are the driver’s height – short motorists find it more difficult to determine correct speed when viewing analogue speedometers – and the thickness of the speedometer’s pointer. Felix believes the total variation in speedometer accuracy, taking all factors into consideration, can be as high as 8km/h at 60km/h and 13-15km/h when travelling at 110km/h per hour.