Here is my formula for calculating trip time on my ET4.
Assuming I'm avoiding every highway I can when riding the scooter:
1) Get the Google estimated trip time for a car (
don't select"avoid highways")
2) Double it.
I've done this for four years now and the results have been very consistent.
Now, back to your own scooter routing...
Do not trust the "avoid highways" setting on Google maps or your GPS unless you are fond of surprises.
I generally plan my trips this way...
1) Use Google and select "avoid highways."
2) Then go over each leg of the trip and compare it to paper maps to eliminate the highways it sends you on anyway*
3) Redraw the map to your new specs.
I find the printed Google directions a bit hard to follow when on the road, so I'll usually write my own set of simpler-to-follow directions. For example, Google will print out every single turn and direction change when all you need to know is follow the signs for route 65W until the next town.
*
OK, you may get lucky and Google really will avoid all the highways. In my experience...it will just avoid most of them.