The tachymeter is a classic component of a racing watch. It adds to the visual appeal of these types of watches. It’s not just a cool-looking feature, though. It can provide some handy information. Let’s take a look at what a tachymeter is and learn how to use one.
What is a Tachymeter?
A tachymeter is a scale found on a watch that measures units per hour. With a tachymeter, you can figure out the speed of something, distance traveled, and productivity.
The scale is almost always found on a bezel or around the outside edge of the watch’s dial. The scale usually starts at the seven-second mark with 500 units but can vary depending on the model and manufacturer. The scale ends at 60, which corresponds to the number of seconds in a minute.
A tachymeter works in conjunction with a chronograph, which is a stopwatch function on a watch. The position of the chronograph’s second hand indicates the reading of the tachymeter scale.
Let’s look at how you can use a tachymeter to measure speed, distance, and productivity.
How to Use a Tachymeter
Measuring Speed with a Tachymeter
To measure speed with a tachymeter, you need to measure something moving between two points. The distance between the two points must be known.
For example, mile markers on the highway or the stated distance between turns on a racetrack would work.
As long as the object being measured can go between the two points in 60 seconds or less, the tachymeter can give you the speed without any calculations.
Let’s assume we want to measure a car’s average speed between two markers with a distance between them of a mile.
When the car passes the first point, start the chronograph (stopwatch). You do this by pressing the top pusher button. When the car passes the second marker, stop the chronograph by pressing the same pusher.
Then, you look at where the chronograph’s second hand stopped and the corresponding reading on the tachymeter scale.
In the image below, you’ll see that the second hand was stopped at 45 seconds. This corresponds to 80 on the tachymeter. That means that the car’s average speed over the mile was 80 miles per hour.
This works for any unit used to measure something. So it could be kilometers, meters, nautical miles, or feet. So, if it’s a kilometer between markers, the car traveled at an average speed of 80 kilometers per hour.
The example we used measured an event that fit within the 60 seconds measured by the tachometer. How do you use a tachometer if it takes more than 60 seconds between markers?
With a bit of math, you can figure out the speed.
Let’s say the car took 1 minute and 20 seconds to travel between the two markers. This is a total of 80 seconds. You can divide this by 2, which gives you 40.
If you go to 40 seconds on the dial and look at the corresponding value on the tachometer scale, you’ll see that it is 90 units per hour.
Now divide that by 2 as well. This gives you 45. So the car had an average speed of 45 miles per hour between the two markers.
You can also use a bit of math to figure out speed in miles per hour over smaller or larger distances.
Let’s say you wanted to measure the speed of a vehicle over a quarter-mile. You time the car and get a tachymeter reading of 350. This indicates the car was going 350 quarter-miles per hour.
To get the full miles per hour, you now must divide the reading by 4. This gives you 87.5 miles per hour.
Now, if you wanted to measure something very fast, like a jet’s speed, you’ll need to do it over a larger distance. Let’s say you could time a jet over 5 miles and get a tachometer reading of 100.
This means the jet traveled at a speed equal to 100, 5-mile units per hour or 500 miles per hour.
Measuring Distance with a Tachymeter
You can also use a tachymeter to measure the distance traveled over a period of time. To be able to do this, the object has to be traveling at a known, constant speed.
For example, let’s say that you’re driving on a highway and your odometer is broken. You set your cruise control to 65 and start your chronograph. When the second hand gets to the 65 mark on the tachymeter, you’ve traveled a mile.
If you’re traveling slower than 60 miles per hour, you’ll need to do a bit of math.
Let’s say you’re going at a constant speed of 45 miles per hour. You have to double this to get a number over 60 that can be found on the tachymeter scale. With 45 miles per hour, this would be 90. So when the tachymeter reading corresponds to 90, you’ve gone half a mile.
Measuring Productivity with a Tachymeter
Because a tachymeter measures units per hour, you can apply this to things outside of distance. For example, you can measure units of a product produced or a defined series of actions that will be repeated over and over.
Let’s say you owned a restaurant. The daily special comes with mashed potatoes, and your cook needs to peel all the potatoes before they can start cooking. To figure out how many potatoes could be peeled in an hour, you could use your tachymeter.
You start the chronograph when the worker begins the process and stop it when they’re done. If it took less than 60 seconds, let’s say 30 seconds, you can use the corresponding tachymeter reading, which is 120. So, that worker can peel 120 potatoes in an hour.
If it takes longer than 60 seconds, you’ll need to do a bit of math. Let’s say it took 90 seconds to peel and chop a potato. You’ll have to divide this by 2 to get 45 seconds.
Look at the tachymeter scale for the corresponding reading at 45 seconds, which is 80. This means the worker can peel and chop 80 half-potatoes in an hour. Divide this by two, and you have 40 whole potatoes peeled and chopped in an hour.
How the Tachymeter Scale is Determined
The tachymeter uses a formula to determine the units per hour value. To find the value at any given elapsed time, you would use the formula below:
The small t in the equation is elapsed time in seconds. The capital T is the value on the tachymeter scale. The 3,600 is how many seconds there are in an hour. As you can see in the graph below, the tachometer value decreases exponentially as time elapses.
Because the chronograph’s second hand makes one revolution, the tachymeter scale typically stops at 60. As you will see, some vintage watches allow for multiple circles of the second hand.
Types of Tachymeters
There are few different types of tachymeters that you might come across. Most tachymeter watches have the scale laid out around the outside of the dial, either on a fixed bezel or around the dial’s outer edge.
There are also versions of tachymeter watches that have the scale swirling around the center of the dial. These are called spiraling tachymeters or circling scale tachymeters. They’re generally found on older, vintage watches.
On these watches, the tachymeter scale winds out from the center of the dial. It can measure slower speeds because it can measure more than one revolution of the chronograph second hand. An example of this is the Longines Heritage Telemeter Chronograph.
Another variation of a tachymeter watch has a rotating bezel. The scale is laid out on the bezel, and the user can use it to measure consecutive lap speeds. Some of the Heuer’s Autavia models from the 70s have a rotating tachymeter bezel.
These are handy for races with laps. It works like a normal tachymeter watch, except that when the first lap is completed, the bezel can be rotated to mark where the chronograph’s second hand was at the end of the lap.
The chronograph continues until the end of the next lap. A new tachymeter reading could be done based on where the second hand was at the end of the previous lap.
Spiral and rotating bezel tachymeter are rare. Almost all the watches with tachymeter scales on the market today have a normal type, with a fixed scale around the outside of the dial.
As you’ve seen, tachymeters are a simple tool that can provide fascinating measurements.
Although they were developed for racing, there are a lot of other real-world applications for these watches. From finding an athlete’s speed to figuring out how productive you can be, tachymeters can provide helpful information quickly and easily.
As a staple feature of the racing watch style, a tachymeter also provides a cool motorsports-inspired look to a watch. To learn about other watch styles available, look at this guide to the types of watches.
There are also a bunch of things watches can do outside of just telling time. If you want to learn more about what other exciting things a watch can do, check out this post about watch complications.