I read above here that GPS has 10-15 feet (3-5 meters) accuracy. But when I zoom into the track that GPS draws on the map, it shows me consistently on the right side of the road, only some places it may occasionally show me as having gone in the ditch or something like that. I think it is quite precise, within maybe a meter or perhaps even a bit less (3 feet or less).

But the GPS measured speed, as when you look at your phone (I don't use cyclocomputer but phone that I sometimes mount on the bars) as on a speedometer, it is or it seems to be laggy. I never get a feeling from it that it is accurate. it probably shows some average speed I had some ten or fifteen seconds earlier.

So now I bought computer that uses those strap on sensors for speed and cadence (not a sensor coupled to a magnet). The 'speed' sensor goes on the wheel hub. I am wondering if it will work in vertical position when it is strapped on the wheel spokes right next to the hub, say hidden behind cassette (I suppose normally it would be on the front wheel?). For one thing, my hubs are conical in shape and also the sensor would look neater if strapped on the spokes.

I guess it depends what's inside the sensor, how it detects the wheel turns. It is probably some mechanical system, like a metal ball enclosed in the sensor body that 'falls' and closes a contact every time it is 'upside down' (or it may be a mercury contact)? This repeated opening and closing of an electric circuit is then broadcasted via BTooth to the computer. However this quote talks something about 'error rate from sensor' which goes against my logic where the computer simply counts wheel turns, each turn one signal 'click'.