Just installed Garmin speed and cadence sensor 2, to a Felt tri-bike I have on an old Blackburn Mag trainer, which is not a smart trainer. When attempting to pair my sensors to the Zwift app, only one or the other seems to pair, but not both. My wheel size is 700×23C and it doesn’t seem like the swift app understands this. At times on a flat road I was paddling 90 RPM in 13th or 14th gear, and the Zwift app was telling me I was going 8 mph at 132 W. One time really pushing it I got to 20 mph downhill. I typically put out 190W-205W and go 20-24mph on flats. I am using a trainer tire, at 40 psi, and have screwed just enough pressure from the mag to/against my rear wheel so that there’s no slipping of my rear tire as I increase cadence and or pressure when pedaling out of the saddle. I am using an iPad (not a iPad Pro) for Zwift. Very frustrating.
I have mine at 110-115 psi, 40 psi is way too low.
Also read this once you get the wattage corrected: https://forums.zwift.com/t/how-does-zwift-determine-my-speed