Made a road trip from AZ to CA this weekend, and made a pretty cool discovery heading down the grade between Chiriaco Summit and Indio on I-10 westbound. As a reminder, I have a Prius IV without dynamic radar cruise, etc. With cruise control set to 72-73mph, I found that the car actually maintained the speed on a 3-5% grade. When observing the Hybrid System Indicator (HSI), I saw the charge meter exceeding 2/3 capacity! The car was doing light braking for me w/ just regeneration while maintaining my set speed (can't say if brakes were actually used though, but regen was for certain). To confirm this, I canceled cruise and noticed no more than 1/4 travel on the meter. Dropped into B mode and had maybe 1/3-1/2 regen on the charge meter w/ ICE spinning. Resumed cruise in "D" mode, and the charge meter went back to 3/4 w/o additional ICE spinning! This never happened on the Gen II - using cruise on this same grade my speed would often exceed my set speed. I'd have to cancel cruise and lightly brake to get the same regen. Other times, I'd drop into "B" mode and apply light gas pedal pressure to balance out the "hold back" to maintain my speed. Gen I was a bit closer as I could use "B" mode w/ cruise and it would modulate the two. This is a great change in how the cruise works. I truly have no more reason to complain that "B" mode doesn't work w/ cruise!
I noticed this on my drive home from the dealer the other day. We have a very steep and long bridge over the Mississippi River Gulf Outlet (the man-made tributary that is responsible for most of Katrina's damage). On my way down the bridge, I was maintaining my set cruise speed as well as regenerating gobs of battery life. That would never happen in my '05. In fact, in the '05, I would have the cruise set at 70mph, and by the time I reached the bottom of the bridge I would be at 76 to 77 mph. This GEN III is full of pleasant surprises.
Interesting find! My cruise (as well as other Toyota cars we've had) usually hold the speed but it depends on the grade of the hill (definitely won't hold if I'm going down a mountain side). It's nice to know that it will now hold the speed on steep grades!
My '06 HiHy picks up speed going down a grade w/ or w/o cruise control and the B is marginally acceptable. The GenIII will actually slow down when backing foot off accelerator pedal (cruise off) and slow down even faster in B - I like this very much over the '06. And I have noted that when cruise is on, speed down grade is maintained at set speed.
My GenIII was out for its first long highway trip over the past week, and its first opportunity to check out this feature on steep mountain grades. It works! On two extended descents, Cruise Control used regeneration until the traction battery was full, then converted to engine braking mode, spinning up the ICE to 3-4000 rpm and pulling a strong vacuum through the intake (as displayed on a ScanGauge). It sounded like a huge vacuum cleaner firing up. Since those grades, I've been using Cruise to get maximum regen from short and variable downhill slopes, and to avoid lighting up my brake lights on hills where the more lossy non-hybrids don't need brakes.
