Somehow hit the sweet spot yesterday during my journey to Chattanooga TN from Charlotte NC. 50/50 interstate/back roads, temps 85-93, AC at 72, negligible wind, no cruise control, moderate traffic, wife, dog and cargo aboard. Shorter in total distance than the all-interstate route, took about the same amount time and a lot more scenic. Over half a tank left at 373 miles after crossing a mountain range and staying mostly at the speed limit. Nuts!
Hard to say for sure because about 50 of the miles driven that day (May 12) were around town prior to the trip, plus since the dog was with us, lunch was ordered/eaten stopped in the car with A/C running (along with most pit-stops), thus our Trip-B average speed was rather dismal. The frame below shows the total for the day inclusive of the leg from Chattanooga to Huntsville (May 12). MPG got even better. May 8 represents the trip from Huntsville to Charlotte that involved a lot more interstate. Still not too shabby.
I've noticed that since the weather has warmed up I've been getting closer to 70 calculated at the pump, good stuff!
How fast were you driving on the interstate? So it does seem the lithium ion does bump up the fuel economy all the way around.
I would estimate an average speed of 60 MPH on the Interstate (posted speed limits were 65 to 70 except for construction zones). Being a mountainous area, my speed going up hill would have been around 50 to 55 (still passing trucks) and around 65-70 going down, so I did not keep a consistent speed in order to eke out the best MPG. Some may see this as OCD behavior but for me it's more of a game to keep me entertained. Maybe therapy is in order? My wife would answer YES!
An update on a subsequent trip from Charlotte to Huntsville on July 7. This trip involved more back roads and less interstate than my previous post above, resulting in steeper ascents/decents, more sharp curves and a lower average speed (but much nicer views!). Temps were on the cooler side for much of the time considering it is summer in the south (but AC running throughout at 74 F) since we experienced heavy rainfall for several hours. Me and one passenger aboard with minimal luggage (plus a half a peck of SC peaches, yum!). If you like to take the scenic route, Prius is for you! Another screen that includes some errands the day before the long drive (zeroed at fill-up).
Just to add one more thing.... There is an amazing sweet spot on the return journey that improved overall MPG. A 50+ mile stretch of Hwy 64 along the Ocoee river is the most Prius friendly gentle down slope you could ever imagine (with views to match). I don't believe the ICE was required but once or twice during that magic stretch. Speed limit of 45 MPH. Although it was only two years ago that I was almost crushed by an 18 wheeler swerving way into my lane on a sharp curve at high speed on that stretch. Reminds me of the old Jack Benny skit where a robber asks him at gun point "you're money, or your life". [long pause] When demanding an answer with a wave of the gun, Jack answers "I'm thinking about it!". lol
I recently did a mountain trip as well over a 7000ft mountain pass. From end to end including some freeway I got 60 MPG for the trip (according to the computer). The latest Prius is just stunningly efficient. In line with your numbers, I think the main thing is speed. The mountain road I took was limited to 45-55 MPH because it was windy so on my way down I didn't need to use gas to keep up with traffic when the road leveled out. If the speed limit was higher, I would have had to burn gas and then waste energy using the friction brakes on slopes.
I bet the 'B' transmission setting for engine braking would have negated the need for friction brakes and would boost your economy up over 100 mpg. You would do that once the battery was fully charged from regenerative braking. I'm pretty surprised to hear of such high economy going over the mountains. What I've seen is that it spikes up quite high going down the side, but then dips down equally low when climbing. Haven't tried it in my Prius yet though!
B mode should only cool and save the friction brakes, not provide any boost to MPG. My experience is that high average elevation improves MPG, thanks to both reduced drag in the thinner air, and (mostly in non-hybrids, not so much in the Atkinsonized Prius) reduced engine 'pumping' loss. The primary problem with mountains is steep descents requiring friction or engine braking, wasting gravitational energy that could have provided nice glides on shallow slopes. With shallow descent angles, the car can often glide well and entirely recoup the extra energy required for the initial climb, and end up with excellent overall trip MPG.
It is a boost in economy because the engine is using hardly any fuel; inertia keeps it spinning. At least that's how it works in conventional engines, as evidenced by the instantaneous fuel economy read-out. I believe the 'B' mode in Prius mimics that, in order to get engine braking.
On these downhills, the Prius engine uses zero fuel, in both D and B modes, so B isn't an improvement on this score. This clearly shows on a ScanGauge-II (or other OBDII-ort engine monitor) fuel consumption display, which isn't capped to the 100 or 200 MPG display limit (depending on model year) of U.S.-market Prii. Modern conventional engines should also be burning no fuel on these downhills, whether downshifted or not, also as shown on the same ScanGauge display. I believe it is labeled DFCO, deceleration fuel cutoff, and I see it in both my present and past Subarus. On shallower hills, one may find conditions where high gear burns some fuel but downshifted does not. But on steeper hills, it should be zero in all practical gears.
