Omega wristwatches (and others) include an outer dial labeled "TACHYMETRE". This web page explains what the dial does and how to use it. The documentation that comes with Omega wristwatches is amazingly absent on the subject.
The TACHYMETRE dial can be used to compute many things, but its primary purpose is to compute a speed after noting how long it takes to travel a fixed distance (like one mile or one kilometer). The dial is a logarithmic scale that computes the function:
TACHYMETRE DIAL = 3600 / Elapsed Time In Seconds
The scale is valid for all elapsed times from 7.2 seconds to 60 seconds. If the duration of the event is outside this range, then the answer on dial is invalid.
For example, supposed you wanted to measure the average speed a race car was traveling. After starting the chronometer function when the car passed the starting line, and stopping it after the car traveled exactly one mile, you noted that the chronometer hand is pointing at the 4 o'clock position, i.e. 20 seconds had elapsed. Looking beyond the 4 to the Tachymetre dial reveals the chronometer hand pointing to "180". This means the average speed of the car was 180 MPH.
Let's say instead of race car speed you are measuring something much slower, like sail boat speed. In this case you need to use a shorter distance, because the elapsed time must fall within the 7.2 - 60 second range. For this example, let's say it took 36 seconds for your sail boat to travel 1/10 of a nautical mile. Reading the Tachymetre dial gives a speed of 100 knots, but we only traveled 1/10 of a nautical mile, so the actual answer is 1/10 of that or 10 knots.
Now lets say you wanted to measure the speed of a very very fast airplane. After traveling 10 kilometers, you noted that 10 seconds had elapsed. The Tachymetre dial gives an answer of 360, but we traveled 10 kilometers, so the answer is 10 times that, or 3600 km/hour.
Now there is really nothing magic about using the Tachymetre dial to measure speed. You can also use it to measure other things, like gas consumption. Supposed it took 50 seconds to burn up a pound of gasoline. Reading the Tachymete dial shows that you are burning 72 pounds of gasoline per hour.