I believe that this stopwatch is calibrated to compute speeds (in MPH) at the Indianapolis Motor Speedway, a 2.5 mile track.

Here’s how the watch works – when you start the timer, both hands will be straight up . . . on the numeral 45. The short yellow hand rotates around the dial once every 45 seconds and the black hand rotates around the dial once every three seconds.

So you start the timer at the start / finish line (or any other fixed point), and after 45 seconds, the short yellow hand has made one full revolution; it’s back on the numeral 45. At the 51 second mark, the yellow hand is on the start of the black section; the black section covers 51 seconds to 54 seconds, which translates to a speed of 175 MPH to 165.5 on a 2.5 mile track (read on the outer printed tachymeter scale, printed in black). At the 54 second mark, the yellow hand will be at the start of the red section; the red section covers 54 seconds to 57 seconds, which translates to speeds of 165.5 MPH to a little under 157 MPH on a 2.5 mile track.

So you basically start the timer, and wait 51 seconds for the tachymeter scale to be “useful” . . . for any car doing a lap between 51 and 57 seconds, the timer will give you its speed in Miles Per Hour.

Perhaps Peter can confirm the rotation of the hands, etc.

Jeff