Using Fenix 6 + Speed Sensor and Stationary Bike (Wahoo or Garmin Speed Sensor)

From what I can see of the Peloton Bike it would be completely meaningless to attempt to measure speed and distance from the flywheel as the resistance can be anything you like and in no way can be calibrated to represent the actual effort vs speed of real life.

You could have little or no resistance, I assume, and spin away like crazy, whisking thin air as though descending a mountain, or turn it up to the max like the toughest climb back up again. There will be no data available for gradient/ascent/descent, so any pretence at simulating and measuring speed will be nonsense.

I've used my girlfriend's spin bike with Zwift, fitting Assioma Duo power meter pedals. Speed and distance from the bike are irrelevant and not even available. Power is what matters, and Zwift will turn that power into simulated speed based on multiple factors such as gradient, surface type, bike and wheel choice, rider characteristics and draft. The speed is not available to the watch and I don't care. I don't train to a speed. I train to a power. The watch has everything it needs to monitor my training (power and heart rate) and Strava maintains my record of speed/distance/ascent/descent/segments as calculated by the Zwift simulation.

As for the treadmill, typically a footpod would be required in order to send speed and other running data to the watch, but once again the elevation would not be factored into the output. Maybe you could use something like this for the treadmill....

https://www.dcrainmaker.com/2020/01/npe-runn-treadmill-smart-sensor-everything-you-need-to-know.html

Personally I use a Stryd footpod. I don't get gradient data, but my treadmill has a fixed incline (Assault Air Runner) and I just have to suck it up when it comes to constantly running up hill and showing a lower pace. 

Another way to look at the bike problem.....

The bike has only one gear. The speed of the flywheel is directly proportional to the cadence of the pedals. The actual effort you expend makes no difference to the flywheel speed, only your cadence affects it.

Let's say you pedal at 80 rpm and you decide that 80 rpm is 20 MPH. What should happen if you increase the resistance but continue to pedal at 80 rpm? On a real bike that would be like changing up a gear and the bike going faster or hitting a slope and the bike going slower.

How about if you reduce the resistance and maintain 80 rpm? That would be like changing to a lower gear where a real bike would go slower, or descending a slope where a real bike would go faster. The flywheel speed won't change so a speed sensor cannot detect a change. The speed sensor also has no idea of your resistance setting. It only knows flywheel speed. It has no way to compute a realistic road speed from the data available.

There is no relationship whatsoever between the speed of the flywheel (or your cadence) and how that would be reflected in the real world. There are far too many variables in the real world that are not replicated on a spin bike.

Now, if the Peloton Bike actually can output a speed via Bluetooth or ANT+ based on its own fancy calculations of power converted to speed then that might make sense, but a 3rd party speed sensor on the flywheel just doesn't have sufficient data. 

I have a Johnny G Spin bike from way back when they first came out. They are built like a tank. Anyway before all these sensors I drilled a hole through the flywheel ,put a longer-ish bolt through it and attached an Avocet bike computer and calibrated it to the circumference of the flywheel and ran  the cadence down to the crank arm. Since Garmin came out with sensors I still use I have those attached as well using the Avocet as control. My Edge 510,530 Fenix 3hr and 6X are all measuring slightly different if I manally put in the same number according to the circumference in the speed sensor.So I have adjusted each number in each Garmin to be pretty close to the Control (Avocet). Based on my outdoor riding I feel I ride the pretty close to the  same distance and speed on my Spinner bike. Give or take a little.  60-80 rpm are hills and 80-110 are rolling to flat depending my resistance I set.