Units Per Hour

[cabinetman]

I've had a few watches with an outer ring with the words "tachymetre", "tachymeter", or "units per hour", and never used it for anything. Ran across this site, and it cleared it up I think.
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[BerniePA]

OK -- I understand that --BUT --What TIME is it!!!

[dbhost]

Wow, a watch, how quaint.

[LCHIEN]

You have a stop watch. You time how long it takes to cover a distance unit, like miles or km. Say using the mile markers on the Interstates highways.
Stop the stop watch when you hit a unit (like a mile). The hand indicates seconds on the normal time dial but on the tachymeter dial it reads the inverse. So if you ran a mile in 30 seconds you would doing 120 MPH according the outer dial. 60 seconds would be 60 MPH. 15 seconds for a miles would be 240 MPH. Turs out the calculation is correct for any units like KM.

Basically, the number on the outside ring = 3600/seconds.
Inverse seconds is rate in units per second, and the 3600 multiplier converts units per second to units per hour.

Looks kind of cool because its a non-linear 1/x scale. Many of those watches just have it for looks, few people but race car drivers and production people would really use it, nowadays there's better tools, maybe none simpler though.