How is the speedometer related to maths?

Answer 1

How this is answered for a person depends on what level of math the questioner is. But here is a basic answer:

The speedometer measures the speed (rate of change of distance traveled with respect to time) at any particular point in time. In Calculus, it would be said that the speedometer shows the derivative of the function for distance traveled.

Note that speed is a scalar, and therefore doesn't depend on direction: distance traveled is not the same as position, since you could drive in a circle and travel a distance of several miles without getting more than a mile from the starting point, for example.