HOW SPEED IS DETERMINED
 

In an automobile or truck, speed is determined by the same odometer that measures distance. However, instead of simply counting up the number of wheel rotations from a given starting point, a speedometer counts the number of wheel rotations in a given period of time. Knowing the wheel circumference, the number of wheel rotations in a certain time interval can be translated directly into meters per second.
You know, of course, that most speedometers respond almost immediately to a change in speed. These instruments measure the rotation rate of a car or truck axle by another method, similar to that used by the engine’s tachometer (a device that measures revolutions per minute, or rpm). A reallife car or truck speedometer measures instantaneous speed, not average speed. In fact, if you want to know the average speed you have traveled during a certain period of time, you must measure the distance on the odometer and then divide by the time elapsed.
In a given period of time t, if an object travels over a displacement of magnitude q at an average speed vavg, then the following formulas apply. These are all arrangements of the same relationship among the three quantities.
q = v_avgt
v_avg = q/t
t = q/v_avg