Today’s lesson in auto parts takes us to the under-appreciated speedometer, that tacky (get it) little arrow that tells us when we are about to get a speeding ticket from a cop with nothing better to do with his or her day!  Let’s face it, without the speedometer most of us would not be driving as we would have had our licenses yanked a very long time ago.

First patented in 1902 by Otto Schulze, the speedometer uses a rotating flexible cable that is linked to the car or truck transmission.  This is actually a fairly complicated little piece of machinery so for the purpose of this article we will simply tell you that it involves a magnet that is attached to the speedometer cable; as the magnet rotates it provides torque to the speedometer pointer as the speed in the vehicle increases.  There are also cups and springs and other cables involved and it all has to be calibrated perfectly so that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer.  One of the calibrations has to do with the diameter of the tires as well as the drive ratio in the differential.

See, all very complicated.  Nowadays there are electronic speedometers which use sensors that are mounted on the transmission.  This sensor delivers electronic pulses which correspond to the rotational speed of the driveshaft.  A computer converts the pulses into a speed and that speed is shown on the digital display on the instrument panel.  This and many other functions/readings are controlled by the ECU which is constantly monitoring the functions in your engine.

For those of you who are considering heading down to your local salvage yard to pick up a speedometer or other used auto part be warned: speedometers are inexpensive but properly installing them is a bit tricky.  Just as it is difficult to explain how a speedometer functions, so is it difficult to properly install one and especially when it is an electronic speedometer.  Although we at All Import Auto Parts would love to see every person in America purchase used replacement parts from us we also feel obliged to point out which used parts are hard to install.

Some recycled auto parts like batteries or radiators or wheels are obviously easy to replace; others like the speedometer or fuel pump are not so easy.  Our best tip: have your sister marry a car mechanic and you will be set for life.