Speed is inaccurate


I am using a Tacx genius. In the Tacx movies speed is like it should be.
However in zwift when I drive in the heaviest gear ( don’t know if that’s correct) and have no problems in clmbing a hill of 3%
I can’t understand why. I turned of my FTP but that doesn’t work. I re installed it again but no luck
So my question is why does my trainer work with Tacx movies but not (as good) in zwift?

I am not following what you are asking. 

Speed within Zwift is determine by watts (zPower would be rear wheel speed and trainers power curve), weight entered, in-game bike used, in-game drafting, and virtual elevation changes.