What is a Tachymeter ?

Answer 1

A tachymeter (also tachymetre or tacheometer) is a kind of theodolite used for rapid measurements and determines, electronically or electro-optically, the distance to target, and is highly automated in its operations. Tachymeters are often used in surveying.

On watches
A tachymeter is also used as the name of a scale sometimes inscribed around the rim of an analog watch. It can be used to compute speed based on travel time. To use it for this purpose, start the chronograph at a mile marker. At the next mile marker, the point on the scale adjacent to the second hand indicates the speed (in miles per hour) of travel between the two. A standard tachymeter will only determine speeds greater than 60 miles per hour using this method; slower speeds can be measured by decreasing the unit of measurement (to half-miles or kilometers, for example).

The speed of an object is defined by the equation
Speed = Distance/Time

The tachymeter scale computes the function
TachymeterDial = 3600/Elapsed Time
.
The spacings between the marks on the tachymeter dial are therefore proportional to 1/t where t is the elapsed time, but because of the somewhat similar pattern and because of people's familiarity with logarithmic graph paper, the tachymeter scale is sometimes also mistakenly described as logarithmic.

The function performed by a tachymeter is independent of unit of distance (i.e. is not specific to miles or kilometers). A tachymeter can be used to measure speed in nautical miles per hour or meters per hour, or to measure an industrial production process in units per hour. A tachymeter is simply a means of converting elapsed time in seconds per X to X per hour.

The scale is generally only accurate for elapsed times over approximately 7 seconds.

Answer 2

If you are referring to the tachometer on a car it measures the RPM which is the revoultions per min. that and engine crank shaft makes. You will usually see it numbered 1 through 9 to 14, but you have to take that number times 1,000 to get the RPM. They are designed so you can see if you are over reving the engine before it is shifted into the next gear.

Answer 3

If we're talking about the same thing (Spelled Tachometer) it is used to measure RPM. (Revolutions Per Minute) Usually found in cars, esp. high performance cars as engine RPM is used to determine when to shift. I hope this helped. Don't worry about the spelling. I actually had to double check it myself.