The computer works just like any other wireless bike-computer, with a fork-mounted sensor that detects a spoke-mounted magnet as it thrum-thrums past and beams the info up to the head-unit on the bars. The difference is in the interface which looks more Gran Turismo* than Tour de France, all analog dials and twisting knobs.
The speed is shown with a needle on a dial and the mileage (or, in this case, kilometer-age) reads out on a retro-style odometer that can be switched from trip-distance to total distance at the slide of a switch. The wheel-size, which needs to be input for this kind of rotation-counting setup, is dialed in via a knob on the magnet-sensor unit.
And now the flaw, although not really a big one. The Bicycle Speedometer has a built-in electronic “bell”, triggered by pulling back on that side lever. The sound would be both a drain on batteries and less loud than a proper metal ding-a-ling model, and the holes to let out the sound would also let in the water.
Ditch the bell and I’m sold. The device is mounted with a leather-covered clip. Classy.