How to Use a Tachymeter Bezel: Step-by-Step Speed Calculation

What Is a Tachymeter?

A tachymeter (from Greek: tachos = speed, metron = measure) is a scale printed or engraved on the bezel or chapter ring of a chronograph watch. It allows you to calculate speed over a known distance, production rate, or any “units per hour” measurement — without a calculator, phone, or any electronics. It’s been a standard feature on racing chronographs since the 1930s and appears on iconic watches like the Rolex Daytona, Omega Speedmaster, TAG Heuer Carrera, and Breitling Navitimer.

Despite being on millions of wrists, most owners have never used their tachymeter. This guide changes that.

How the Scale Works: The Math

The tachymeter scale is based on a simple formula:

Speed = 3600 ÷ elapsed seconds

Since one hour = 3600 seconds, dividing 3600 by the number of seconds gives you the “units per hour” rate. The bezel simply pre-calculates this division for every second from 7 to 60:

• At 60 seconds: 3600 ÷ 60 = 60 (the scale starts here)
• At 30 seconds: 3600 ÷ 30 = 120
• At 20 seconds: 3600 ÷ 20 = 180
• At 12 seconds: 3600 ÷ 12 = 300
• At 7.2 seconds: 3600 ÷ 7.2 = 500 (the scale ends here)

The scale only works for events lasting between ~7 and 60 seconds. For longer events, you use fractions of the measured distance.

Step-by-Step: Measuring Speed on the Highway

1. Identify a known distance. Highway mile markers are perfect. As you pass one, start the chronograph by pressing the top pusher.
2. Drive at steady speed. Maintain your speed until you pass the next marker (1 mile later).
3. Stop the chronograph. Press the top pusher again when you reach the second marker.
4. Read the bezel. The chronograph’s seconds hand points to a number on the tachymeter scale. That number is your average speed in miles per hour.

Example: You start at mile marker 42. You stop at mile marker 43. The chronograph reads 36 seconds. The seconds hand points to 100 on the tachymeter scale. Your average speed: 100 mph.

Example 2: Same distance, 45 seconds elapsed. Seconds hand points to 80 on the scale. Average speed: 80 mph.

Using Kilometers Instead of Miles

The tachymeter doesn’t care about units — it measures “units per hour.” If your known distance is 1 kilometer, the reading is in km/h. If it’s 1 mile, the reading is in mph. If it’s 1 nautical mile, the reading is in knots. The math is identical.

Beyond Speed: Measuring Production Rate

The tachymeter works for any “units per hour” calculation. Factory example:

1. Start the chronograph when a machine produces one widget.
2. Stop when the next widget comes off the line.
3. Read the tachymeter: the number is widgets per hour.

If the machine produces one widget every 24 seconds, the tachymeter reads 150. Production rate: 150 widgets per hour.

Measuring Distances Over 1 Unit

For distances greater than your base unit, divide the tachymeter reading. Example:

1. You time a car over a 2-mile stretch. Elapsed time: 40 seconds.
2. Tachymeter reads: 90.
3. Divide by 2 (because the distance was 2 miles): 90 ÷ 2 = 45 mph.

For a 10-km distance with 50 seconds elapsed: tachymeter reads 72, divide by 10 = 7.2 km/h (walking pace).

Limitations of the Tachymeter

• Events under 7 seconds: The scale doesn’t go above ~500, so events faster than 7 seconds can’t be measured directly. For very fast events, use a longer distance.
• Events over 60 seconds: The scale starts at 60 (= 60 units/hour). For slower events, use a shorter distance or multiply your base unit.
• Accuracy depends on human reaction time: Starting and stopping the chronograph introduces ~0.2-0.5 seconds of error each way. For casual use this is fine; for professional timing, electronic chronographs are used.
• Non-linear scale: The tachymeter is logarithmic — markings are closely spaced at high values and widely spaced at low values. Precise readings above 200 are difficult.

Which Watches Have Tachymeter Bezels?

• Rolex Daytona: Engraved on the Cerachrom ceramic bezel. The most famous tachymeter implementation.
• Omega Speedmaster: Printed on the anodized aluminum bezel ring. The Moonwatch’s tachymeter was used to calculate spacecraft speed during Apollo missions.
• TAG Heuer Carrera: On the inner chapter ring (not the bezel), keeping the dial clean.
• Breitling Chronomat: On the inner rotating bezel, allowing the rider to align the scale for different starting points.
• Seiko SSB031: Budget chronograph with a fully functional tachymeter at ~$100.

Frequently Asked Questions

Does the tachymeter work without the chronograph?

No. The tachymeter reads the position of the chronograph seconds hand — if you don’t start the chrono, the scale is meaningless. You need a watch with a working chronograph function to use it.

Can I use a tachymeter while driving?

Technically yes, using highway mile/km markers. However, operating a watch while driving is distracting and potentially illegal in some jurisdictions. Have a passenger operate the chronograph, or pull over to read the result.

Why does the scale start at 500 and end at 60?

Because 3600 ÷ 7.2 ≈ 500 (the fastest measurable event at ~7 seconds) and 3600 ÷ 60 = 60 (the slowest at exactly 1 minute). These are the practical limits of the tachymeter scale.

Is the tachymeter useful in 2026?

For actual speed measurement, your phone’s GPS is more accurate. But the tachymeter remains useful for quick “back of the napkin” calculations, production-line monitoring, and — honestly — as a conversation piece. It’s one of the few remaining analog instruments that actually does something.

Which DR.WATCH models have tachymeters?

Our Rolex Daytona collection features the classic tachymeter bezel with working chronograph movements. The scale is faithfully reproduced from the original Cerachrom design. Browse our best sellers for chronographs from multiple brands.