On your (analogue) wristwatch, is there a funny dial around the outside, which reads 60 at the top, and goes around backwards, 65, 70, 75, 80, 90, 100, 110, 120, up to 300 or 400 (near 2 O'clock)?
It's called a tachymeter, and it's designed to help you to estimate the speed you are travelling, provided there are mile posts, or kilometre posts by the side of the road (or track)
As you pass one marker post, start the stopwatch. Stop it as you pass the next mile (km) post. Look at the elapsed time. Perhaps it reads 45 seconds. Look across to the tachymeter dial, and that tells you 80 miles (km) per hour.
It is--in theory at least--that simple. In practice, things are more difficult. I remember travelling on a Deutsche Bahn express train alongside the Rhine, where there are kilometre posts marking the length of the river, but it was really hard to hit the stopwatch consistently, to get a reliable speed reading. Also, since the train speed varied during each kilometre of travel, the readings were not that useful anyway.
But for everyone who wants to know how to use their super sports chrono-with every gizmo under the sun, that's how ya do it.
This node written, formatted and edited in Dann's E2 offline scratchpad