Toyota Prius: Hills and Gas Mileage

"Dick Byrd" <com> wrote in message
news:KrqQg.1940$.. 
Dick, wind not-with-standing we are just moving a weight from one altitude
to another. An engine's efficiency becomes a factor to the extent that they
are usually more efficient under heavy loads because factors like internal
friction are less significant at full load. If this is also the case with
the Prius engine, then it would be better to hold a constant speed drawing
power from the engine on the uphill side where the load is maximize.

I've observed truckers doing what you describe. I believe they do this in
situations where their engine doesn't have sufficient power to maintain
speed while pulling their load on the uphill side, not to achieve better
fuel efficiency.

Bill wrote: 

You're right. They are usually just trying to get a "running start" at
an approaching upgrade.

As for driving on hills in the Prius, I think you are also correct. My
experience is that I really don't see any mileage degradation with
speed changes on up or down grades. You can save some fuel driving
slower on upgrades, but that is the same on flat ground - you tend to
save fuel by reducing speed.

My latest experience with driving my Prius in the hills was just this
weekend. Sacramento to Reno and back.

Sacramento to Reno is a net climb of about 4,500 feet over 115 miles.
Of course you have to go over Donner Pass which is just over 7,000
feet. I normally would not try to make my car maintain 70 mph for the
entire trip, but this was the first time I made the trip in the Prius
and decided to see it do just that. It held 70 mph the entire trip.
That little engine was really working on the steeper parts of the
climb, and the batteries went to two purple bars over Blue Canyon and
got down one bar going over the summit just before Boreal. But even at
70, we got over 30 mpg the summit. Coming down into Truckee and then
into Reno the numbers came up for an average of 44 mpg on the trip up.
I ain't going to complain a bit about that - My Chrysler Pacifica gets
about 18 mpg on the same trip.

Coming home it was bit harder to maintain 70 mph up to Truckee because
of traffic. And the westbound climb up the Donner grade took the
batteries down to one bar before the summit. We were staying up with
the fastest traffic and chanting, "I think I can. I think I can. I
think I can," as we waited to see what happened if you get to below I
purple bar. It is a big net decent from Donner to Sac. Average
mileage on the way home was 59.8 mpg. I was predicting 60, but to
borrow a phrase from Maxwell Smart - "Missed it by that much."

Curtis:
I really appreciate your input on this. Even though I live in the Wash.
D.C. area, I also drove that Sacramento-Reno trip about a month ago in my
2006 Prius.
My question about slowing down on an upgrade is not just to save gas by
going slower, because I am speeding up a corresponding amount on the
downgrade so as to keep the same average speed. The question is will the
Prius do better overall if you want to maintain 65 mph average, if you: (1)
Stay at 65 mph uphill and downhill; OR (2) speed up with a light throttle to
75 mph on the downgrade and then let the car slow down to 55 mph on the
upgrade with a trailing throttle?
Dick

******************************************* 

"Dick Byrd" <com> wrote in message
news:mU2Ug.14485$.. 
Dick, I live among the rolling hills of Minnesota's lakes country. A couple
of my regular routes are about 40 miles of nothing but rolling hills. Along
both I set my cruise at 55/56. It's very difficult to gauge the effect
these hills have because wind is such a significant factor but over a couple
of years it's become clear that the net change in altitude is almost as
significant as the wind. This is consistent with the laws of physics. Your
example increases and decreases speed by the same amount nullifying that
factor. The only remaining variable is engine efficiency and, like I said,
engines are generally more efficient under heavy load. If that
generalization applies to the Prius (I have no reason to believe it doesn't)
you would be better off maintaining a constant speed in order to achieve
maximum efficiency on the uphill side.

Bill:
It may be as simple as you say. Certainly the ICE is more efficient when
producing more power (up to a point). However, try my hill strategy on your
rolling hills of Minnesota and watch your MPG indicator. In fact, set your
throttle opening by that indicator. Here is the comparison.
(1) If I travel at 65 mph downhill and uphill: I get 100 mpg+ on the
downgrade because it is throttle-off or throttle very-light. Then as I have
to push the throttle hard to maintain 65 mph on the upgrade, mileage drops
to 18 - 20 mpg. Overall for the downhill-uphill, about 40 - 45 mpg.

(2) If I use very light throttle downhill, I can show 70 - 80 mpg and
run up to 75 mph. Then on the uphill, I back-off the throttle and allow the
speed to decrease, keeping the fuel consumption at 50 - 60 mpg. Overall for
the downhill-uphill, about 55 - 60 mpg.

Try it and let me know how it works for you. Of course, the actual mpg
and mph figures will vary widely depending on how steep and long is the
downgrade and how steep and long is the upgrade.

Dick
*******************************************

"Bill" <now> wrote in message
news:com...