How to tell how fast a car is going?

No, the simplest solution would be to stop. Speed = 0; no computation or sensors necessary!

JM

Speed != velocity.

Quite a lot of cars use a signal off the *alternator* to drive the tach because precise accuracy isn't important. But once ECU control of ignition timing became standard the tach was driven by the ECU which gets its information from a sensor on the crankshaft.

Aftermarket tachs usually get their signal from the ignition.

Definition of "tachometer" is not restricted to "engine". Its an RPM meter. A speedometer is nothing but an RPM meter calibrated in MPH or KPH.

Classic hand-held tachometers had a wheel or shaft which one might press against the outer rolling surface wheel-to-wheel, or press against the center of the rotating axis if accessible. Of course wheel-to-wheel wouldn't work today as those who would use such tools do not understand the math to calculate actual RPM or understand the difference in the two means of applying the tool.

There are myriad other ways of doing it. One I'm familiar with is that used is that used by the R/C aircraft people who measure the propeller speed optically - the tach divides by two (or three, for a three bladed prop) to get the prop speed. Since there is usually a 1:1 correlation between propeller and engine speed (or if there is a gearbox fitted, it is only a single ratio type) it is easy enough to obtain engine speed as well.

Another method comes to mind from a similar source. I remember reading an article "Using a piano as an audio tach". The idea was to find the note on a piano which most closely matched the tone of the engine exhaust. The RPM was obtained by looking up that note in a table. Apparently you could get quite accurate results in this manner.

Gives a new meaning to tuned pipes...