How do we know the meter did not speed up?

Because of its mechanical design, the meter cannot speed up. The meter is a positive displacement meter. This means that the meter will only register if water passes through it. A set of magnets transmits the motion of an internal disc, causing the numbers to turn only when water is passing through the meter. As a meter ages, it tends to slow down (measuring less water than actually passes through it) or stops altogether. If this happens, the city removes and replaces the meter.