Driving 20 mph slower over a 3 hr drive is 60 miles. It’s 190 mile to my youngest brother’s, mostly on the motorway. Normally it takes 3/3.5 hours. say it takes 3 hours that’s an average speed of 63.33mph. I set the adaptive cruise at 78mph, if i was to set it at 58mph it would add 2 hrs to our journey there and back, 8 hrs instead of 6 hrs. I don’t think I could do it as we only stay for a few hours.