Tachymeter calculator

How a tachymeter works

The scale is one calculation printed on a bezel: 3600 divided by the elapsed seconds. Time a vehicle over exactly one mile, stop the chronograph at 45 seconds, and the number under the hand is 80, meaning 80 miles per hour. The scale has no units of its own, which is why it works equally well for miles, kilometers or widgets off a production line.

It is printed from about 7.2 to 60 seconds, because below 7.2 seconds the numbers crowd past 500 and above 60 seconds the chronograph hand has gone around once and the reading no longer means what the scale says. For anything slower, time it over a longer distance and multiply, which is exactly what this calculator does for you.

The calculator defaults to miles because most of its readers drive in them, but the arithmetic is identical either way and the panel prints the other unit alongside. Eighty miles per hour is 128.7 km/h; time one kilometer in the same 45 seconds and the answer is 80 km/h, which is 49.7 mph.

A worked example, over a measured mile

You are a passenger on a US interstate with mileposts. You start the chronograph level with milepost 82 and stop it level with milepost 83. The hand stops at 50.0 seconds.

1. The tachymeter arithmetic is 3600 / 50 = 72. That is the number printed under the hand on the bezel.
2. Because the distance was exactly one mile, the reading is the answer: 72 mph.
3. Converted, 72 × 1.609344 = 115.9 km/h.
4. 50 seconds falls inside the 7.2 to 60 second window, so the printed bezel would have given the same figure without any arithmetic at all.

What you should conclude is narrower than it looks. You have an average speed over that mile, accurate to about the quality of your thumb, and nothing more.

Notice that the scale is not linear, which is why the numbers on a bezel crowd together at one end. Ten seconds of difference at the fast end, from 15 to 25 seconds, is worth 96 mph (154.5 km/h). The same ten seconds at the slow end, from 50 to 60 seconds, is worth 12 mph (19.3 km/h).

Timing it well enough to trust

Because speed varies with one divided by time, the percentage error in your answer is simply the percentage error in your timing. Two pusher presses, each carrying a tenth or two of human reaction time, can easily add up to 0.4 seconds.

• At 50 seconds, 0.4 seconds is 0.8 percent: 72 mph plus or minus about 0.6 mph (1.0 km/h). Fine.
• At 15 seconds, the same 0.4 seconds is 2.7 percent: 240 mph plus or minus about 6 mph (10 km/h). The short timing is the unreliable one, which is the opposite of most people's intuition.

Three other things spoil a reading. The distance has to be exact: mileposts are placed to a nominal one mile spacing and a car odometer is worse still, so neither is a laboratory instrument. Start and stop with the same part of the vehicle level with each marker, or you have built in a car length of error. And the speed has to be steady, because a tachymeter reports the average: brake once for traffic and the number describes a journey nobody made at any single moment.

Read the central chronograph seconds hand, not the running seconds subdial, and check the minute counter before you trust anything near the top of the scale.

When the distance is not one unit

This is the mistake the printed bezel invites and the calculator exists to prevent. The scale assumes you measured exactly one unit. Time a quarter mile in 15 seconds and the bezel reads 240, but your speed is 240 × 0.25 = 60 mph (96.6 km/h). The bezel is wrong by a factor of four and gives no hint of it.

The rule is: read the scale, then multiply by the distance you actually covered. Two miles in 100 seconds gives 3600 / 100 = 36, times 2 = 72 mph (115.9 km/h). That one is off the printed scale entirely, since 100 seconds is past a full sweep of the hand, but the arithmetic is unchanged. Enter the distance in the second field and the tool applies it. Leave it blank or at zero in speed mode and it assumes one unit.

Using it in real life

The honest answer is that almost nobody times a mile on a highway anymore, and that is fine. The tachymeter survives because it is the visual signature of a chronograph and because the underlying arithmetic still counts anything repeatable: laps, rows of stitching, parts off a line, breaths per minute on a pulsometer variant of the same scale.

Rate mode is that second use. Say a part comes off a line every 24 seconds. 3600 / 24 = 150 an hour, 60 / 24 = 2.5 a minute, and 150 × 8 = 1,200 across an eight hour shift. Set the target field to 160 and the tool reports you behind; set it to 140 and you are ahead. In rate mode the second field stops meaning distance and becomes that target, and it defaults to 60 per hour if you leave it empty.

Two practical cautions apply to the chronograph itself. Do not reset the chronograph while it is running unless the watch is documented as a flyback. And never operate the pushers underwater unless the manual explicitly allows it, which for pushers it almost never does: the ratings defined by ISO 22810 and ISO 6425 are tested with the pushers undisturbed.

What the scale cannot do

The calculator prints a decimal place. That is arithmetic precision, not measurement precision, and the two are not related. A mechanical chronograph hand can only stop on a beat of the balance, so its true resolution is one eighth of a second at 28,800 vph and one tenth at 36,000 vph, which is what high beat chronographs were sold on. Our beat rate converter gives the figure for your own caliber. In practice your thumb, not the movement, sets the limit.

• A watch with a tachymeter and no chronograph. Plenty of dials carry the scale as decoration. With no hand you can start and stop, it is a printed pattern.
• Base 1000 dials. Some scales, including several Speedmaster dials, are marked for a base of 1000 units rather than one, so they are read over 1,000 meters or yards rather than one mile. Read the dial text before you read the scale.
• Spiral and snail scales. Printed on the dial rather than the bezel, these wind through more than one revolution and so cover intervals longer than 60 seconds. The 7.2 to 60 second rule does not apply to them.
• Long intervals. Past 60 seconds the seconds hand has lapped and the bezel reading is ambiguous. Use the minute counter and the calculator.
• Telemeter scales. A relative, calibrated on the speed of sound rather than distance: roughly one kilometer per three seconds, or one mile per five, from about 1,125 feet per second (343 meters per second) in air at 68F (20C). It moves with temperature, so treat it as an estimate.

The decision the number should drive is usually a small one: whether the pace you are keeping matches a plan. If the answer changes something expensive, use an instrument built for the job. Our guide to chronographs covers what the complication is genuinely good at, and the Valjoux 7750 page covers the caliber under most of the dials that carry this scale.