Although vehicle Australian Design Rules (ADRs) did not require a speedometer to be fitted up to 1989, roadworthiness requirements have always required they only have to be ‘operating’ if they are fitted.

A functioning speedometer and odometer is now a roadworthiness requirement, yet the accuracy of the instrument is not called into question.

However, in theses days of “Safety Cameras” around almost every corner, it would be prudent to ensure a speedometer accuracy test is conducted on your vehicle if you have any doubts at all.

Inaccurate speedos mean that many motorists are unaware they are speeding. Vehicles fitted with larger wheels may render a speedometer inaccurate. While roadworthy regulations limit increases in diameters of modified wheels, not all motorists who install larger wheels are aware these do fewer revolutions to achieve the same speed. Unless the speedo is calibrated, it could be registering 50kmh while the car is actually clocking up 60kmh. The motorist could be caught and fined and the car declared unroadworthy because the tyres are 15mm larger than the originals.

Also low tyre pressure could also lead to inaccurate readings, as could the load being carried or fitting of non-standard or modified gearboxes and differentials& Even worse, many cars are simply fitted with inaccurate speedometers.

Vehicle manufacturers must comply with Australian Design Rule (ADR) 18/01 Instrumentation, which specifies vehicles should be fitted with odometers that are accurate to +/-10% above 40kmh. At +10%, a speedo will register 50kmh when the car is actually travelling at 55kmh, and 100 at 110kmh.

Keep in mind that a speedometer can go unchecked for the life of a car, regardless of the number of registration transfers, because roadworthy tests do not demand speedos be tested for accuracy.