That’s interesting that it’s so far off.

I use a Garmin 235 watch, it has occasionally shown me double the gain. I know it will do this this so I don’t pay much attention to it. It works entirely from gps data.

After the ride, I sync to Strava and the data seems to get averaged based on their databases. I’ve always accepted this as accurate enough for my purposes.

I think that using wheel magnets for speed and barometer for altitude has been done since the early 90’s (Avocet 50). The technology still exists and is probably available now wirelessly.

I personally like having a single watch since I have 4 bikes. To me that’s worth a slight decrease in accuracy.