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Re: Hills and Gas Mileage Dick Byrd 09-21-2006
Posted by Dick Byrd on September 21, 2006, 2:38 am
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From many years ago, I remember reading that the best mileage for
highway driving could be obtained by speeding up with moderate throttle on
downgrades and then slowing up using trailing throttle on the upgrades. This
was explained, quite untechnically, that the car can better "take the gas"
going downhill whereas it is a waste to push down hard on the throttle going
uphill.
I've been doing this where feasible (i.e. not driving the motorists
behind me crazy!). Typically, when trying to average 65 mph, I'll speed up
to 75 mph on the downside and gradually slow to 55 mph by the crest of the
uphill. The MFD seems to indicate that this is a good strategy - BUT - I am
concerned in that vehicle speed and power are related by a
cube-relationship. Is my speeding up to 75 mph going to use so much more
power that I'd be better keeping a constant speed at 65 mph?
Dick




Posted by Bill on September 21, 2006, 1:49 pm
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> From many years ago, I remember reading that the best mileage for
> highway driving could be obtained by speeding up with moderate throttle on
> downgrades and then slowing up using trailing throttle on the upgrades.
> This was explained, quite untechnically, that the car can better "take the
> gas" going downhill whereas it is a waste to push down hard on the
> throttle going uphill.
> I've been doing this where feasible (i.e. not driving the motorists
> behind me crazy!). Typically, when trying to average 65 mph, I'll speed up
> to 75 mph on the downside and gradually slow to 55 mph by the crest of the
> uphill. The MFD seems to indicate that this is a good strategy - BUT - I
> am concerned in that vehicle speed and power are related by a
> cube-relationship. Is my speeding up to 75 mph going to use so much more
> power that I'd be better keeping a constant speed at 65 mph?
> Dick
>
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.



Posted by Curtis CCR on October 1, 2006, 11:08 pm
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Bill wrote:
> > From many years ago, I remember reading that the best mileage for
> > highway driving could be obtained by speeding up with moderate throttle on
> > downgrades and then slowing up using trailing throttle on the upgrades.
> > This was explained, quite untechnically, that the car can better "take the
> > gas" going downhill whereas it is a waste to push down hard on the
> > throttle going uphill.
> > I've been doing this where feasible (i.e. not driving the motorists
> > behind me crazy!). Typically, when trying to average 65 mph, I'll speed up
> > to 75 mph on the downside and gradually slow to 55 mph by the crest of the
> > uphill. The MFD seems to indicate that this is a good strategy - BUT - I
> > am concerned in that vehicle speed and power are related by a
> > cube-relationship. Is my speeding up to 75 mph going to use so much more
> > power that I'd be better keeping a constant speed at 65 mph?
> > Dick
> >
> 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.

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." :)


Posted by Dick Byrd on October 2, 2006, 3:06 am
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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

*******************************************
> 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." :)
>



Posted by Bill on October 2, 2006, 12:52 pm
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> 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, 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.



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