I found some good instructions for how to use a tachymeter scale on a chronograph from www.righttime.com.
Q. What is a TACHYMETER (TACHYMETRE) scale and how does it work?
A. WHAT IT DOES:
The tachymetre scale can be used to compute many things, but it's primary purpose is to compute the speed after noting how long it takes to travel a fixed distance (e.g. one mile or kilometer). The dial is a scale which 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 the dial is invalid.
HOW TO USE IT:
For example, suppose you wanted to measure the average speed a racecar was traveling. After starting the chronograph function when the car passes the starting line, and stopping it after the car travels exactly one mile, you note that the chronograph hand is pointing at the 4 o'clock position (i.e. 20 seconds have elapsed) Looking beyond the 4 to the Tachymetre dial reveals the chronograph hand pointing at 180. This means that the average speed of the car would be 180 MPH.
Let's say, instead of the race car speed, you are measuring something much slower, like sailboat speed. In this case, you need to use a shorter distance because the elapsed time must fall within the 7.2 - 60 seconds range. For this example, let's say it took 36 seconds for your sailboat to travel 1/10 of a nautical mile. Reading the tachymetre dial gives a speed of 100 knots. However, since we only traveled 1/10 of a nautical mile, the actual answer is 1/10 of that or 10 knots.
Now let's say you wanted to measure the speed of a VERY, VERY fast airplane: after traveling 10 kilometers, you noted that 10 seconds have elapsed. The tachymetre dial gives an answer of 360 but we traveled 10 Km. Therefore, the answer is 10X360, or 3600 km/hour.
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. Suppose it took 50 seconds to burn up a gallon of gasoline. Reading the tachymetre scale shows that you are burning 72 gallons of gasoline per hour