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Keep an eye on your speed
The speedometer, or "speedo" for short, is the central instrument that helps you keep track of the current speed of a vehicle. It's a very important source of information and helps you adhere to the legal speed limit, as well as keep the vehicle under control. In addition to the factory-provided speedometer, there are also some very interesting solutions for retrofitting.
Speedometer operating principle
The speedometer is a standard part installed in all vehicles. It consists of a sensor, a signal line and an output device on the car's dashboard. Speed sensor fall into one of two classes, analog and digital devices. Although from a technical viewpoint, digital speedometers are hardly a challenge these days, the analog speedometer remains the standard factory choice in most vehicles. The so-called "eddy-current speedometer" is most common. A small additional gear connected to a magnet is rotated by the drive. This transmits an angular momentum to a second shaft by means of magnetic force, which is however hindered from rotating along by a spiral spring. This shaft is connected to a pointer, which is calibrated via a scale. The faster the magnet rotates, the higher its magnetic pull. Thus, the faster the vehicle moves, the further the pointer is deflected. The advantage of this simple principle is that it is technically simple and inexpensive to produce.
Electro-mechanical speedometers work with a permanent magnet attached to the vehicle's drive. A sensor measures the rotational speed and the control unit uses this information to calculate the speed of the vehicle. The measured value is then indicated on a display. This can be a digital display, although a manual display is more common with electro-mechanical speedometers.
Advantages and disadvantages of speedometer types
Eddy current speedometers require just a few components and function purely mechanically. However, they are not completely accurate. If the speedometer cable breaks, it immediately fails and must be completely replaced.
An electro-mechanical speedometer depends on a control unit to convert the angular velocity at the drive to the speed of the vehicle. Yet, it is far more precise. Since these speedos feature fewer mechanical parts, their lifetime is much longer. Speedo defects Speedometers only know two states: They either work as intended, or not at all. In eddy-current speedometers, the cause of a failure is usually a broken speedometer cable. The permanently changing speeds result in permanent torsion in the cable, which leads to fatigue and later to breakage of the material. In this case, replacing the cable is the only option. In electro-mechanical speedometers, the permanent magnet can come unstuck from the drive or corrosion at the sensor contacts can disconnect the signal line. Yet, if a speedometer displays incorrect values, there's usually a different cause: In this case, it is likely that the vehicle was equipped with tires that are not approved for it. The information provided by the speedometer is always within a tolerance range to allow for different tire sizes. According to legislators, up to 10% plus 4 km/h more are permissible. Therefore, the speedometer usually indicates a slightly slower speed. Yet, if using tires that are too big or too small, the speedometer significantly deviates from the real speed. One should also always be careful when installing replacement speedometer from junk cars. The speedo models can even greatly vary among the same type of vehicle. This, too, can easily result in incorrect speed indications. Replacing a speedometer In modern vehicles, replacing a speedometer is less complex than you might think. On most cars, the display panel can be removed without removing the steering wheel or dashboard, which makes replacement far easier. In most cases, a speedometer can only be ordered from the manufacturer as a replacement part. In contrast, the accessories trade offers optional rev-counters, which can easily be coupled with the speedometer. Rev-meters or retrofit speedometers can be purchased from the trade. Prices start at 150 euro.