Hi. Got a dumb trainer with garmin speed And cadence sensors both ant +. At beginning of ride and flats, both are detected and show similar values to my fenix 5s+. But here we go… up for the road and down for the speed, I mean real down. From about 8% I clock an average of 3kph(on swift, garmin shows good value) I do reciprocate the gradient by changing manually the resistance of the trainer but even with lowest resistance and cadence of 120 I just fail to fall of my bike! Flat & down goes back to normal…
This will help you understand it better: https://forums.zwift.com/t/how-does-zwift-determine-my-speed
Also, what trainer, what trainer selected on Pairing Screen, sensors, tire size selected on Pairing Screen.
DO NOT MANUALLY CHANGE THE RESISTANCE ON YOUR TRAINER, KEEP IT AT THE SETTING WHICH IS DISPLAYED FOR THE TRAINER ON THE PAIRING SCREEN.
Tip, ignore the speed that the Fenix is displaying, it will hardly ever match Zwift.
You said you are on a dumb trainer so you should do the opposite when climbing. Since your trainer isn’t providing resistance you should shift to a harder gear to up your watts whereas those with a smart trainer have that feedback applied by the trainer so they usually shift to an easier gear.
If you keep the same wattage on the flats as you do on an 8% grade you will always go way slower up the climb.
That makes sense, will try this morning.