Speed Can also by construed as your rate of beers pulled per hour (thanks DSIO ) as well as miles or kilometres travelled per hour The following pictures were nicked from a post by jmsrolls (Fr. John) over on the rolexforums and explain the uses of the various bezels available on the Speedmaster much better than I could.
First, I don't have a Speedy. A pair of Seamasters yes, but no Speedy. Second, I apologize for the long-ish-ness of the story to follow. The first watch I ever owned that cost more than $100 was a TagHeuer chrono (quartz movement) that I bought on a business trip to Switzerland in 1995. In addition to a rotating diver's bezel, it has a tachymetric scale printed around the rim of the dial. Not long after I bought the watch I was on an East-West flight over the western United States. Through the plane's the window I could see rural roads laid out on 1-mile squares, in accordance with the Range/Township/Section division of land which is common out west. Bored by the long flight, I decided to try estimating the plane's speed by using the tachymetric scale on my watch. I picked out a particular pockmark in the glass on my window to use as a sighting reference. When the pockmark crossed the eastern boundary of a 1-mile square, I pressed the stopwatch button on my wrist watch, and when my pockmark crossed the western boundary of the same square, I pressed the button again. Amazingly, the sweep second hand had stopped right on the "500" mark of the tachymetric scale, meaning that the plane's speed was 500 mph. It seemed a little like magic that my watch could do that