Ever wondered how does a speedo work? Simply put, your car dashboard is receiving signals straight from the wheels to measure the speed on the speedometer. Speedometers are innovative devices that use electromagnetism to send signals from the physical wheel speed to the dash display.
Every time you get stopped on the road, and the police ask you ‘what speed were you going at Sir’ you must know that, on an average, your speedometer is showing you 7-8 points more than your actual speed. If your speedometer shows 100 Km/h, you might be at something around 92! The how and why behind this is quite interesting from legal and technical standpoints and to learn the reasons, we need to go back a little in history.
Standard speedometers have been keeping a check on driving speeds since the early 1900s. The very first standard mechanical speedometer, also called the eddy-current speedometer was patented and later developed by Otto Schulze, an inventor from Strasbourg. With cars travelling as fast as 30 Km/h during that time, Schulze invented a device that would help motorists keep their car speed at bay. Its funny to imagine that people were scared to travel at 30km/h.
Speedometers in the 20th century used to have two separate dials. While the smaller dial was for the motorist to check and adjust speed accordingly, the bigger one was for the police for monitoring purposes. Today, we have moved to digital speedometers – an instrument cluster or a digital dash that shows an electronic readout of the speed and various other elements on the dashboard screen.
How do speedometers work?
In a mechanical setup, a speedometer uses analogue devices that attach a drive cable directly from the transmission to the dial. Electromagnetic current is used to calculate the car speed by measuring how fast the cable attached to the drive shaft is rotating. The electronic speedometers simply use speed sensors instead of a physical cable to calculate the vehicle speed.
Why are the mechanical ones called ‘eddy-current’ speedometer?
The cable that connects the wheels to the speedometer is long, flexible and made of twisted wires. The speedometer case contains a magnet that is connected to the cable and hence starts spinning as the wheels start moving. The magnet is placed inside a hollow metal body called the speed cup. The speed cup is attached to the pointer of the speedometer. As the cable rotates, the magnet starts spinning inside the speed cup creating a fluctuating magnetic field. This creates electric currents inside the cup. Since these currents have nowhere to go, they start swimming around in swirling eddies, hence the name eddy-current.
How Accurate Are Speedometer Readings?
It has been found that most speedometers exaggerate speeds to keep motorists in check by displaying a slightly higher speed, so may not actually be driving as fast as you think.
The diameter of your car wheels will change the reading on your speedometer accordingly. Consistent usage, pressure and wear often lead to the wheels getting underinflated causing the speedo to produce a higher reading. According to the updated speedo compliance regulation ADR 18, your reading cannot be slower than the actual speed, which is why cars are calibrated in a certain manner.
The same law states that ‘over indication of accuracy must never depict more than 10 points of actual speed’. This means if your car is running at 100 km/h, the speedo can never show 99 km/h. But it also means, a car could be travelling at 100 km/h, but legally, the speedo will display as high as 110 km/h.