Two weeks ago I was in Hamburg. The maps here do not show the speed limits on the truck except in the highway marked spots.
Instead tomtom maps show them all the time. I refer to the maximum legal speed for trucks. Here the result and the difference of calculation.
I have experimented personally.
But being a professional, we can make an average of 80 km / h and we'll have the time we need to travel a certain distance. I do not think the program calculates badly. Calculates based on the information it holds.
Sorry for my English, I hope I was understood.