Toyota Safety

Imagine for a moment the variety of sensors located in the dashboard of a typical vehicle. Oftentimes, a vehicle’s dashboard, or instrument cluster, resembles a kaleidoscope of lights and gauges monitoring everything from fuel level to engine temperature. While specific gauges found within different dashboards may vary, there is a prominent feature that remains constant in all vehicles regardless of make or model. What could this feature be? The answer is simple: the speedometer.

The speedometer is arguably the most important gauge within a vehicle's instrument cluster. Its primary function is to indicate the speed of one’s car in miles per hour, kilometers per hour or both. While the purpose of a speedometer seems rather straightforward, its design can vary. In fact, there are two types of speedometers: mechanical and electronic. Mechanical speedometers, also known as eddy speedometers, require the collaboration of several vehicle parts to work properly. In order to measure speed of a vehicle, one must be able to measure the rotational speed of either the wheels or the transmission and send that information to some sort of gauge. In most cars, measurement takes place in the transmission. Once this information is gathered, it is the job of the drive cable to interpret the data.

The drive cable consists of a number of tightly wound, helical coil springs wrapped around a mandrel. In most vehicles, the drive cable is fed from the transmission to the dashboard that houses the speedometer. Basically, the drive cable is connected to a set of gears in the transmission, meaning when the vehicle moves, the gears turn the mandrel within the drive cable. This motion communicates the rotational speed of the transmission down the length of the cable to the speedometer, which dictates the driving speed to the driver.

Conversely, electronic speedometers (which are relatively new to the automotive world) do not require the use of a drive cable to interpret speed data. Instead, an electronic speedometer receives its data from a vehicle speed sensor (VSS). The VSS is mounted to the transmission output shaft or to the crankshaft and consists of a notched metal disk and a stationary detector that covers a magnetic coil. As the teeth of the metal disk move past the coil, they disrupt the magnetic field, creating a series of pulses that are sent to the vehicle's central computer. From there, speed is determined from the input pulse frequency.

While both mechanical and electronic speedometers produce trustworthy data, no speedometer is 100 percent accurate. In fact, most manufacturers build speedometers so they fall within a fairly narrow tolerance range, no more than one percent to five percent too slow or too fast. As long as a car is maintained at factory specs, its speedometer should continue to register vehicle speed within this range. But if a car is modified, its speedometer may need to be recalibrated.

In summary, speedometers are invaluable assets to drivers. They help monitor one's driving speed, which in turn helps one to avoid speeding tickets. More importantly, they help ensure the safety of drivers and passengers by prompting drivers to stay within legal driving limits. Mechanical or electronic, speedometers are not zipping out of cars anytime soon.