Soldato
If it's busy on a motorway I find it a lot more difficult/tiresome to do 70 than to do maybe 80-85. 80-85 means I can cruise in the outside lane, mostly overtaking people in the inside/middle (but pulling in when it's clear of course!) If I'm doing 70, I'll be pulling in, out all the time. If that makes sense. It's at night when I can pop the cruise on at 70-80 and relax, occasionally overtaking the odd lorry
If you can do 80 mph its not a busy motorway.