On a Mac running BaseCamp, it's only possible to adjust the overall default speed, using a sider base, not individual speeds per road type as was the case in Mapsource when I ran it on a PC.
Mine, for some unknown reason is set as default plus 7 mph, a remarkably exact figure. I find that a route calculated in BaseCamp will give an estimated total time of longer than the estimated time when the same route is transferred and then displayed on my Nav V; the Nav V's estimation of time that'll be taken being invariably lower. I can't pretend that I have ever paid much attention to the times estimates, preferring my own rule of thumb for the day on a day-by-day basis.
Just out of passing interest, perhaps... I sent 17 routes for my Germany jaunt to my Nav V by dragging the file straight into the Nav V (not its memory card) using BaseCamp. I then asked the device to import all 17 routes at once into Trip Planner, which it duly did. I always check the routes visually against those in BaseCamp just to be sure that no bizarre glitches have occurred. I simply check: Does the route look right, des the mileage match, are the waypoints I want as waypoints and those that I want just as shaping points correct and is the time estimation right?
This time ALL the time estimates were way out; a journey time that I know will take 3 hours was estimated to take 10 or more, but everything else was fine. I shut the device down and rebooted, still the time estimates were very wrong. I deleted the routes from Trip Planner and this time imported them within the device, one by one, instead of all at once. This time the time estimates were all correct. I can only guess that the device struggles doing too much at once.