Guessing you'll need traffic info for that to improve. Think both Garmin and TomTom with traffic info will make a better estimate and if possible reroute to get around it.
I went in and selected segments of the road and set an average speed based on experience, but I think if a road has speed limits thats overriding it. Selecting not to use speed limits will often make it even worse. And I cant really just set one average for each road type as thats not working in Norway at all. Our "trunk" and "primary" roads are often narrow and winding in some places and then wide and straight in others. TBH I wish there was a minimum required standard for a road to be in the E route network. We would probably loose half of our E routes :)
A way to use gpx traces for the app to set average speeds would have been nice. No idea if that would even be possible to do.
I would like to know whether traffic lights (OSM: highway=traffic_signals) are considered during calculation - they can decrease the average speed significantly.