I have a 2008 BMW 328i Touring (Station Wagon) with a 6-speed manual transmission (had to special order it while I was in Germany). My daily commute goes over the H-3, from Kaneohe, past Pearl Harbor, then up the H-2 to Schofield Barracks. I drive from about 150’ above sea level to just over 1000’, then back to sea level, then up to 750’, in about 30 miles. A lot of 5% - 6% hillage. Every once in a while it rains hard, but we don’t get any snow here at all … (The “Interstate” freeway’s ‘H-1’ instead of ‘I-1’ 'cos there’s no other state to interconnect to).
The cruise control on the BMW holds my car to within 1 mph, up and down hill,just like it says in the owner’s manual. If I reset it to a slower speed, it will use the brakes to slow the car down. On the BMW you can set a numerical value, so you know exactly what it’s set for.
My wife’s 2005 Mazda3 (stick shift) also does well with cruise control, but I think it’s just engine braking.
Ennyway, on some cars you can use cruise control up & down the mountain - you just have to pay attention, of course, but when is that not true?
Thanks for the details. Good posting.