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Yeah, OK. If your preferred speed is within a few mph of 80, it's very easy to accidentally exceed 80. Enough said.
When I set my cruise control to 79 mph on an Interstate my speed does not go beyond that even on downgrades.
Once your cruise control is on, of course it's not hard to maintain. But you can't even set it at all with trucks passing, and its not worth setting unless it is going to remain set for several miles, which is not very likely.
If trucks are passing then you are probably going less than the speed limit in any case.
I use cruise control in some cases on lower speed local roads. If I look ahead and see a mile or so of potential usage on, say, a 35 mph street, I let the cruise control do the work. Give my foot a rest but keep it near the brake pedal in case I need to brake.
I drove a very new Jeep Compass (slightly more than 4,000 miles on it) for work yesterday to Paintsville, on the fairly gentle grades of the Mountain Parkway and US 460. With a new, modern vehicle like that, I'd easily pick up 5 mph going downhill with the cruise control activated. Engine braking was nonexistent.
Rather poor cruise control, IYAM, at least on a freeway. Above, 2016 Buick Lacrosse. My previous vehicles had similar results (2003, 1994).
Now on a 7% downgrade on a 2-lane highway, that might not hold, but there we are talking about a 55 mph speed limit, and I can downshift from 6 down to 3 and get it to hold the set speed fine.