I never thought about the sensor tech in an avocet, but a hall would just require a single magnet just like a reed switch. I used my avocet for years, the ring eventually got to be too much of a pain and I stopped using it. I hope I threw it out, but it might be in a junk box somewhere.

GPS speed is good enough for me, I know it's unreliable, but close enough. Don't the new ones use an accelerometer?