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Discussion Starter #1
hello all --


i do not have a prius, but am thinking of getting one. i test drove one last night at the local dealer, but was unable to take it on the highway to do some "real world" testing.

some background info. i live in fort worth, but work in dallas (means ~100mile roundtrip). i would like the prius because of its high MPG.

while i found the test drive on city streets to be adequete, i am wondering about what the highway performance would be? from reading the other posts, i see that a lot of people aim for best mpg, so they may drive a little slower (i.e. the speed limit). can anyone tell me how the prius does when you drive it more aggressively, say at 75-80mph? what kind of fuel mileage do you end up with?


thanks for any input, i am also looking at the 2004 model (more cabin space, more power...)


tommy
 

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High speed mileage

It has been my experience that the average mileage for any sustained speed is not a regular function. It is something like this:

Code:
       | s < 20       55
m(s) = | 20 < s < 40  55 - (s - 20)*(7/20)
       | s > 40       303/sqrt(s)
 

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High speed mileage

Mileage goes down with higher speeds. My own experience is that mileage goes down 5-10 mpg between 55 mph and 75+, though I think this may be partially attributable to throttle adjustments at higher speeds.

The mileage can be maximized by driving a little slower than traffic and keeping the cruise control on. Speeding up to pull out and pass a slower vehicle uses a considerable amount of gasoline at 75+, and this severely affects mileage. Finding a slightly slower speed that matches traffic, or results in only a few cars passing you, will allow you to keep the cruise control on and improve mileage significantly.

On a recent 125 mile trip on a relatively flat stretch of highway, I averaged approximately 40 mpg at 75+ mph, while repeatedly slowing down and speeding up to overtake slower vehicles, in moderate traffic. The return trip was at 73 mph in the late evening. Since traffic was very light, most cars passed me; but I was able to keep the cruise control on for almost the entire trip. MPG jumped to 52 mpg, and the difference in travel time was only about 10 minutes.

An additional plus, the drive was much more pleasant without the stress of constantly fighting traffic.
 

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Discussion Starter #4
aron and phoenix --


thanks for the responses.

aron -- how did you come up the with formula? did you characterize from your driving data, or is this published somewhere?

phoenix --

yes, i expected the fuel mileage to decrease with increasing speed. my driving route is 45 miles one way, which usually takes about 1 to 1.25 hours, depending on traffic. also, this is driving between dallas and fort worth, where maybe the center part (20mi?) could be done driving with cruise control -- the rest of the time, there would be too much traffic to be able to cruise.

my current car is a 5-spd nissan maxima. it is so flexible at speed, that i nearly always in the left hand lane, to follow the people going 75-80 mph, and i am able to consistently get 26-27 mpg (92k on the car). i *really* have toned down my speeding, but it still feels horribly slow to be doing 65mph. back when i was doing 12+ hour days on top of the 2hr. daily commute, speed was a necessity (at least that is my excuse). but, perhaps, i need to readjust my driving habits for more economy and peace of mind.... :D

any other useful info that you can share about owning one of these hybrids? i have been reading the posts on this forum, but am always looking for more information...


thanks,

tommy
 

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Re: High speed mileage

Fortunately, this formula does not work for me.
Last weekend I got 47mpg on a 400-mile roundtrip, almost all highway, load 3 adults and 1 child, very light a/c use, mostly steady speeds between 65-75 mph, rare overtaking, outside temp around 65F.

AronRubin said:
It has been my experience that the average mileage for any sustained speed is not a regular function. It is something like this:

Code:
       | s < 20       55
m(s) = | 20 < s < 40  55 - (s - 20)*(7/20)
       | s > 40       303/sqrt(s)
 

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This formula is my best "human" approximation of my observed experiences. Don't worry the fuel economy would be far superior in Dallas where the land is flatter, hotter and less... well... New Jersey driving combat zone. Although, I cannot say that my experiences driving in Dallas were wonderful. I have a 2001 and I have been unable to get higher than low forties mpg at sustained at 75. It takes a lot of prediction work in order not to accelerate or decelerate very much, that is how you get better milage. I guess my formula was off a bit, I never get below 37 mpg. hmm.

Aron
 

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Discussion Starter #7
i've been thinking about this more, and i think the important thing that i want to be sure of is that if i need or want to drive with the speeders, i can do it, and when i am not rushed and want to maximize, then i can just cruise.

now that i think about the terrain, my drive between dallas and fort worth is very flat. wouldn't that mean that i won't be able to take much advantage of the regenerative feature? there are small ups and downs but i would imagine that i would need to keep my foot on the gas the whole time to keep speed? how much deceleration is there when the regenerative part kicks in when i let go of the gas?

i suppose i could increase mileage at the higher cruising speeds by drafting behind the numerous big-a$$ SUVs that clog the roads here? i wonder how close to them i have to be?

i think i would be happy with a mid 40s mpg for my drive, but i'm sure once i actually have one of these cars, i would be playing the "how high can i get my mpg" game too :)

thanks again to all for giving me info.

tommy
 

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80 MPH = MPG?

I love the MarvMobile in the driving I do; I drive 65-70 on interstate highways I see 46MPG fairly consistant.

I don't feel the Prius has the guts to do the aggressive 75-80+ driving you describe. Really can't get out of your own way if trying to pass at 80. We have a ten year old low mileage V-8 Olds Aurora which does 28-30 MPG on highway with plenty of pedal to be able to move in and out of traffic safely but.... It too at 80+ drops mileage MPG to 26MPG. Where I can touch the gas pedal and zip to near a hunderd and back off when required passing. Prius isn't designed for that -- I feel.

I dont feel I can move the Prius at 80+ when some one is blinking their lights behind me.

Disclaimer: I have only done short sprints in the Prius on the Garden State Parkway to see how it handles at speed. During these sprints I see lower 40's even upper 30's MPG (on the display lines). Little fellow is lot of noise not much go!
 

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Higher speed

It looks like the 2004 Prius might fit your requirements better. It remains to be seen how much more oomph it has.
 

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Regeneration on flat terrain

tommiboy said:
now that i think about the terrain, my drive between dallas and fort worth is very flat. wouldn't that mean that i won't be able to take much advantage of the regenerative feature?
You don't really get any regeneration at highway speeds unless it's a very steep downhill. Otherwise, the engine and/or motor is required to do some pushing at all times to overcome friction and drag.

Regeneration is mostly hype. The actual amount of energy saved is not significant--most electrical power comes from the engine spinning the motor as a generator. For example, a typical 5-minute block of city driving may have a single "gold star", representing 50 Watt-hours of electricity created by regenerative braking. This equals 3 kW-minutes, which is 180 kW-seconds. The main drive motor draws a maximum of 33 kW, so this one "gold star" will power the motor for about 5.5 seconds at full power, or 11 seconds at half power. I usually get much more than 11 seconds of stealth mode in 5 minutes of city driving, so the rest of the electricity comes from the generator.

For highway driving, the Prius' great mpg comes from a low drag design, a small gas engine, and not going too much over 60 mph. (Those who understand have claimed a theoretical "sweet spot" of approximately 60 mph for the Prius' gas engine.) I typically drive 69 mph in a 65 mph zone, and I typically see 48 mpg. I have noticed this is heavily affected by wind (despite the Prius' low drag); a headwind or tailwind can subtract or add 5-10 mpg to a trip. In fact, those who drive in hilly or mountainous areas claim terrain has little effect on mileage.

Douglas (2002 Silver, Wisconsin)
 

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Highway mileage

Just a few more mileage calculations. A recent trip from Phoenix to San Francisco resulted in a round trip mileage of 40 mpg @ 80 mph.

Also, the Prius goes great in the mountains. We have a couple of grades where the semi's are going about 25-30 mph, and the Prius will hold 75 mph up the grade, easily. The electric motor torque is good at holding the speed, but acceleration is a little lacking. If you try to accelerate, like from 45 to 75 going up the grade, the ICE is at full throttle and you've got to watch the rear view mirror pretty close for other cars driving up your back.

Steep mountain grades at highway speeds give drastically different results between uphill an down. Uphill mileage can drop to the high 20's for short periods, but downhill is fun. On the trip down from Flagstaff to Phoenix (about 125 miles), the Prius got 75 mpg. One vacation last year, we drove from Ruidoso to White Sands NM (over 50 miles) with the graph showing 99.9 mpg. You just can't beat the mileage, when the ICE is not running!
 

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Discussion Starter #12
yes, i am interested in seeing the 2004 model. if it has acceleration like a 4cyl camry, then i think it will be a contender. i also like the fact that it will have some sort of cargo carrying capacity (for carrying stuff like a mountain bike inside).

thank you again for all the info. i do have more questions now..

if there is no problem keeping 75mph while going up a grade, doesn't that mean that for my driving, once i get up to 80mph, i should be able to stay there for the duration of my trip (traffic allowing, of course)?

so if i understand correctly, the speed issue is not whether the car can get up to and stay at a specific speed (say 80mph), but rather the time for acceleration to get there?

leave it to an engineer to get all excited at the technology in a not-so-high-powered car... :)


no one jumped on my comment about drafting... i know that driving too close to another car (tailgating) is dangerous, but it seems everyone does it in dallas/fort worth. and with the size of these SUVs, i wonder if anyone has noticed how close that have to get to see some MPG improvements from drafting? i know that when i am behind a 18 wheeler, i can feel a turbulent spot at a certain distance from the back of the trailer, and then it gets smoother when pull a little closer. anyone?


thanks again,


tommy
 

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Re: Drafting with the Prius

Out of curiosity, a few months ago, I tried a few mild attempts at drafting with my Prius. I noticed a 1 to 2 mpg increase when I did it while following tractor trailers at 65 to 70 mph. I did not tailgate, but did try to stay close. If the road was straight, and there was no wind, there was a spot behind the truck where this would work. Others, who have followed really close, have reported bigger increases, but that is too dangerous.
I also noticed the same increase in mpg by driving next to the trucks. There seems to be a spot on each side of the truck, near the back. Under the same conditions, no wind and straight road, it worked just as well and was much safer!
 

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maintaining speed

tommiboy said:
if there is no problem keeping 75mph while going up a grade, doesn't that mean that for my driving, once i get up to 80mph, i should be able to stay there for the duration of my trip (traffic allowing, of course)?

so if i understand correctly, the speed issue is not whether the car can get up to and stay at a specific speed (say 80mph), but rather the time for acceleration to get there?
Yes, there should be no problem maintaining 80 mph. It will take you longer to get there than other cars, and you will have less passing acceleration at that high of a speed, but you can stay there indefinitely.

Those who have studied and modeled the Prius have concluded that it has a maximum sustainable speed of 84 mph. The maximum short-term speed is 100 mph (assuming level terrain and no wind, either of which can push you faster or hold you back). However, at speeds above 84 mph, the supplemental electrical power required by the motor to hold the speed is greater than the amount of electrical power that can be generated by the engine while it is providing enough kinetic power to hold the speed.

These calculations were performed on the THS found in 2001-2003 models. Undoubtedly, the 2004 Prius will have a higher sustainable speed. It will also have better acceleration to 80mph and better passing capability at those speeds.

Douglas (2002 Silver, Wisconsin)
 

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Re: Drafting with the Prius

citicar1976 said:
Out of curiosity, a few months ago, I tried a few mild attempts at drafting with my Prius. I noticed a 1 to 2 mpg increase when I did it while following tractor trailers at 65 to 70 mph. I did not tailgate, but did try to stay close. If the road was straight, and there was no wind, there was a spot behind the truck where this would work. Others, who have followed really close, have reported bigger increases, but that is too dangerous.
I also noticed the same increase in mpg by driving next to the trucks. There seems to be a spot on each side of the truck, near the back. Under the same conditions, no wind and straight road, it worked just as well and was much safer!
From a flow dynamics point of view, the drafting gets exponentially better as you get closer. If you can get right in behind him you can actually be "pulled" by the significant pressure drop from the eddies in the wind flow, ie, less then regular air pressure. Unfortunatly this is dangerious. That's why I want a computerized cruise that uses radar to bounce off the vehicle in front of me so the computer can react to changes in the trucks speed, braking as required automatically. Since humans have a reaction time of 1/2 to 3/4 of a second and a computer is reletively instantanious. I see this happeneing with computer controlled autopilot and radar adaptive cruise control.

I know from experience in speed skating that drafting someone can make a HUGE difference in power output required to maintain speed. From short little trials on a very long stretch of plain road on the Mass Pike I can get in behind a semi and only barely touch the accelerator in my honda civic to maintain around 75.
 

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Drafting

Setting aside the safety aspects of drafting, and just considering the physics, would the semi driver be a little perturbed?

Presumably, the energy to keep your car moving at a steady speed has to come from somewhere, and since it's not coming from your car, it must be coming from the semi. So, I expect any increase in your mileage would come at the expense of a resulting drop in the semi's mileage.

However, my knowledge of aerodynamics is shaky. Is it possible that the poor aerodynamics of a semi (flat rectangular back end) mean that the semi has already lost that mileage? And therefore, it would really be "free" energy to the drafter?

Douglas (2002 Silver, Wisconsin)
 

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Re: Drafting

I have read that drafting benefits both vehicles. It seems that two vehicles, which are very close together, have less wind resistance than when they are apart from each other.
In the case of a semi and Prius, I would guess that the Prius would get most of the benefit from drafting.
 

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I think this can be compared to a situation where you're cycling in a group rather than alone. A compact group of cyclists can ride significantly faster than if they were separate using the same amount of energy ; that does not mean that there is any energy creation, but that in the "bad" case some energy is simply wasted in the air, mostly heating it by friction. The same with migratory birds, too.
 

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The low pressure behind the semi will actually increase the drag on the semi, by interrupting that low pressure zone by putting your car in it, you are actually reducing his drag so thereby increasing his milage.

The benefits favor both.

Now we just need some computers to do it.

From speed skating, whether the leader or follower or in the middle of a pack, it is always advantagious to skate in a pack then alone.
 

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Yep, it's the same reason golf balls have dimples, to break up the low pressure area trailing a sphere. A big, flat tail has an even larger low pressure system. A low pressure system 'pulls' - like the aerodynamics of an airplane wing, the low pressure is on top of the wing.

The problem for the truck driver is that he doesn't know a small car is back there, he does know if another truck is following him.

My basic theory of life is the do unto others thing. I hate being tailgated, so I don't do it to others.
 
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