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.
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.