Earlier quoted context omitted.
The score goes from 0 to 140 so that there's some room for when the computers will get faster. When we started working on this, all browsers were maxed at 140, so the computation got changed.
I thought the front page goes to 140 just because it is modeled after actual GM dashboard speedometers produced ~1960-1990, sometimes having range 0-85 MPH, or 0-140km/h in metric markets.
The speedometer graphic was inherited from Speedometer 2. When Speedometer 2 was released, scores were in a reasonable car-speed range. The combination of hardware and software improvements meant that early versions of Speedometer 3 (which includes a subset of Speedometer 2 tests) were consistently scoring above 140, so we adjusted the scaling factor (IIRC, by ~20x) to give plenty of room for future improvements.