Reading a recent issue of the British auto magazine "Evo," I came across two uses of "ton" that I understood from context to mean 100, or more specifically 100 miles per hour. I don't have the magazine handy, but it said something like "...the car took 5.2 seconds to reach 60 mph and 10.5 seconds to reach the ton." Another article used the word in a similar manner. What is the source or etymology of this use of "ton"? Is it merely idiomatic?