As an ex-drag racer (car), I can tell you this: your elapsed time and mph are not necessarily in proportion to each other. The ET starts when you break the light at the start, and ends when you break the finish light. The mph comes from a set of lights a certain number of feet (memory fails me here, lol) before the finish line, to a certain number of feet after the finish line. At least, that's how it was done 35 years ago !