A tachymeter is on the bezel of a watch and can measure speed based on time traveled over a fixed distance. The allows the conversion of elapsed time, in seconds per unit, to speed in units per hour. The tachymeter scale and measure times from approximately 7 seconds to 60 seconds.
A tachymeter scale typically starts at the 7 second mark at 500 units of speed but some models can be found starting at 6 seconds and 600 units or 9 seconds at 400 units. The tachymeter scale can also be found on either the bezel or as an internal scale around the outside of the dial. Mathematically speaking, the equation to calculate is T=3600/t (who said we would never need to use algebra again!). The “T” is the tachymeter scale value, “t” is the elapsed time and 3600 is the number of seconds in an hour. Depending on what you are measuring there may be more math involved in calculating the speed.
To measure speed, you’ll want to first designate your measurement whether it is miles or kilometers. After you designate the measurement, you’ll also want to make sure that you have an accurate representation, such as the distance between two mile markers while you’re riding down the highway. In the following example, we are using a distance of 1 mile between two points.
- Start the chronograph when you pass the first marker
- Stop the chronograph when you pass the second maker
- The seconds hand traveled around the dial to :45
- On the outer point of the dial or on the bezel, this lines up with 80
- This means that we were traveling at a speed of 80 miles per hour
We can also use our algebra equation and calculate the same answer: 3600/45=80.
You can also use the same principle to determine how much work can be completed in one hour. If it takes 20 seconds to chop an apple, you can chop about 180 apples in one hour. This is because the tachymeter reading is 180, 3600/20=180.
Now, to make things difficult, you can still measure something even if it’s over 60 seconds. Lets say it takes us 100 seconds to box up a watch to ship. This is the same as saying that it takes us 50 seconds to box up 1/2 of a watch. This 50 seconds is still within the normal valid range of the tachymeter scale. As a result, you can calculate that 72 half boxes can be completed in one hour based off of where the 50 second mark lines up on the scale, or 36 full boxes per hour. Remember 3600/50=72 and 72/2=36.
If we haven’t overwhelmed you with math just yet, you can also use the tachymeter to measure distance. First, you’ll need to know how fast you’re going, then you can calculate your distance in a similar fashion. Using our speed from above, we can determine that after starting the chronograph when the seconds hand passes the 80 mark on the scale, we know we’ve traveled one mile. This is useful if you no longer have mile markers or another way to measure distance but you know your speed.
This calculation only works if you’re traveling at speeds over 60 regardless of your unit of measurement, due to the scale ending at 60. If you know you’re traveling at a lower speed, you’ll want to multiply the speed by 2. Once you reach that speed on the tachymeter scale, you divide the distance by the original multiplying factor. This means, if you’re traveling at 50 miles per hour, you multiply by 2 giving you 100 miles per hour. After starting the tachymeter and having it reach 100 on the scale, you divide the end distance of 1 mile by 2, giving you a distance of .5 miles traveled.
Regardless of how you’re measuring distance, your speed will need to stay a constant.
Tachymeters are useful to measure both speed and distance. They get easier to use with more practice, so don’t let a little math get in the way of using this function on your watch. The tachymeter scale isn’t just a random set of numbers engraved on the bezel or outer ring of the dial. We hope that you’ve learned how to use the tachymeter correctly and can start using it even if you’re not into racing! Have something other than a tachymeter scale on the outside of your watch? Read our guide on different types of bezels!