The first speedos were elaborate mechanical devices that used a cable to take information directly from the drive shaft to the speedo display.
Then came electronic speedos, which generate electric pulses with sensors attached to the drive shaft. The pulses travel along wires and are read by electronic counters in the display. The more pulses in a set time, the faster the shaft is rotating, which allows the vehicle’s speed to be calculated.
These speedometers are easier to build into a car and are more accurate. The electronic signal enables the speedo to give a digital reading, though most car companies prefer to use an analogue dial – but these are prone to reading errors.
In recent years, speedo accuracy has improved further, using wheel-speed sensors on each wheel averaged out to provide a more accurate display of speed than cars using a single sensor on the drive shaft.
This increased accuracy spurred a tightening of the Australian Design Rules which now prohibit under-reading, and allow over-reading by 10 per cent of actual speed plus 4km/h. This means that if your vehicle’s actual speed is 100km/h, the displayed speed is permitted to be anywhere between 100km/h and 114km/h. Before 2006, speedos were allowed to be out by up to 10 per cent high or low.
This tolerance, particularly the pre-2006 one of 10 per cent plus or minus, has led some to wonder why the road laws don’t allow the same margin of error when it comes to speeding, since we rely on speedometers to stay within the limit. The design rule is a theoretical allowance, not a reflection of the real world. Speedos were always designed to over-read, almost invariably.
Car manufacturers can check speedo accuracy at the end of the production line by putting the vehicle on a calibrated rolling road device and logging the speedo electronically. It is possible to pay a specialist to do this for you. A rudimentary alternative is to use the speed-check devices on some freeways. Simply sit on a fixed speed when passing them to get a rough idea of how accurate your speedo is.
As modern speedometers are sealed units they are not adjustable. But some factors that affect speedo accuracy, such as worn tyres, can be fixed.
Same rule for trucks
The speedometers in cars and trucks are subject to exactly the same requirements in the Australian Design Rule. Some trucks have a speed limiter, which limits the top vehicle speed as referenced by the number of wheel rotations per kilometre to no more than 100km/h. The speedo itself, however, will not typically be any less or more accurate than a car speedo. If anything, you might expect custom-built trucks to display more variation (within the allowed tolerance) than a mass-produced car.