magnet inside the speedometer’s case which is driven by a flexible cable
(Figure 6), usually from the vehicle’s transmission or transfer case
output shaft (Figure 4). On some vehicles, though, such as early
Volkswagens, the flexible shaft was driven from a front wheel. As this
magnet rotates within a field plate, it pulls on a metal “speed cup” and
draws this cup in the direction of rotation against the resistance of a hair
spring. The faster the magnet rotates, the stronger the pull on the speed
cup, and the farther the speedometer needle is drawn around the
speedometer’s face. As the vehicle slows down or stops, so does the
rotating magnet, and the hair spring pulls the needle back down around
the speedometer’s face, or returns it to zero. Naturally, the tension of the
hair spring has been precisely calibrated to offset the pull of the rotating
magnet so that the speedometer reads accurately at any speed.
Figure 2
The odometer, which may include a resettable “trip odometer,” is
driven by a gear mechanism from the rotating magnet shaft (Figure 5).
A mechanical tachometer works exactly the same way, and its
odometer is called the “engine hour meter.”
Now we know what goes on inside our speedometers; yet, it still
surprises me that some very good mechanics seem to think that