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Most efficient speed for best MPG?

Discussion in 'Gen II Prius Fuel Economy' started by Swamibob, Jan 24, 2006.

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  1. Swamibob

    Swamibob Unemployed Member

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    Does anyone know what the most efficient speed for the 2006 Prius is? I know running off the battery would be the most efficient, but I mean as far as highway driving for long stretches, what is the speed I should set my cruise on to get the best MPG? This would be on level ground and assume no headwind or tailwind.

    Thanks,
    Swamibob
  2. tumbleweed

    tumbleweed Senior Member

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    What I have observed is generally speaking slower is better. Of course at 0 mph we get 0 mpg and at some speed, say 70 mph at 65 degrees F, we might get 48 mpg. Well we know we can beat both of those numbers and I expect your question is at what speed does the graph peak out. I think it will depend on a lot of things even if the terrain is dead level with no wind, such as: temprature, tire pressure, use of heat and air conditioning, and a bunch of other things. But in any case I expect the best mileage will be at some speed, say 30 or 35 mph, that is really to slow to drive for long distance. Best thing is to experiment when you can.

    BTW driving on the battery is not always the most efficient way to get down the road, remember that all of the energy in the battery has to be put there by burning gasoline. For example, if you come down a hill and put a lot of energy in the battery you had to get up the hill to start with. The battery is great though because it lets you recover some energy you would otherwise throw away.

    Check some of the older topics in this forum by efusco and others. There is really a wealth of good information here.

    Edit: I should add that using energy in the battery is indeed a good idea and if you can do it with the ICE off so much the better. It's just that you shouldn't drive with storing energy as a goal.
  3. Jack 06

    Jack 06 New Member

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    One of the "mad scientist"-type engineers who sprang up when the Gen 1 Prius was first getting analyzed on the Yahoo groups produced a grid predicting MPG at all speeds, and taking into account factors such as ambient temperature and wind speed. I don't remember where that might actually reside, but I'll bet either john 1701a or Dr. Fusco does. Seems the "sweet spot" was somewhere between 50 and 55, but I'm not sure. If john or Dr. Fusco don't see this and respond, I'll PM one of them. Some other analysis may have emerged since then, anyway, that I'm not aware of.
  4. efusco

    efusco Troll Slayer Staff Member

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    The 64mph "Sweet spot" once proposed on his site is not true.
    The best MPG will be on level terrain b/w 30-40mph. Lower forces the ICE to run too much in a low efficiency state. Higher and drag becomes the bigger factor. It's a linear thing and there's nothing magical about 64mph or 53mph or any other number.
  5. kDB

    kDB New Member

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    i agree, though for me anywhere between 35 and 50mph give me 55+mpg on near-flat streets. i'd say around 42 would be a good place to start testing. but when you start adding in head/tailwinds, hills, etc... it makes it hard to define an accual number.

    and driving on battery only usually isn't a good idea to get better milage. sure it will look like a huge increase while your doing it, but not later when your burning gas to replace the charge. generally i only use ev mode at under 15-20, when i want to burn off a green charge, or when i know i have a long downhill coming soon that will recharge me to green.
  6. tomdeimos

    tomdeimos New Member

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    The best mpg is on battery only at around 30 mph.
    You should easily achieve over 70 mpg.
  7. kDB

    kDB New Member

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    well if your talking instantanious mpg battery would be better, but you have to get that electricity from somewhere, and usually it's from burning fuel.
    but even if you are talking instantanious, simply gliding without battery or gas would be best. i guess it's all in what terms you use.

    the way i read it made me answer this way. if i were to use a whole tank of gas on a nascar style oval track. what speed would be best to drive at with constant pressuse on the pedal (i.e. trying to keep as close to the same rate of gas flowing into the ICE at all times). now since i don't have the money to rent a track to test on (i wish i did, we could answer this once and for all, well at least until you start throwing in pulse and glide, etc.), i answered what i usually average driving to work and back.

    now sure you can argue that you have the regenerative braking, but even then there are limit to where it helps. overall, i try to not charge the battery, but i also don't try to use the charge much either. and as you can tell from my tanks (click link in sig), i can do fairly well. there are only a few times i have averaged below 50mpg on a tank. there are the first few tanks because of me learning how to deal with the problems filling the tank. and three others. one at 49mpg, and two at around 45mpg. the two at 45 were me trying my hardest to drive the car like a normal person would, with the heat up fairly high (normally i don't use heat unless it's below 30 outside, and then just enough to get the cabin to around 45).
  8. tomdeimos

    tomdeimos New Member

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    The battery is why the Prius works. The more you can use it the better. It is completely useless to try not to use it. You can't at 40 mph or below. The car will not let you.

    The only reason for having a hybrid is to benefit from using as much battery power as possible. It is way more efficient and the 70 mpg I mentioned is the min mpg I can achieve daily if I drive slow, which is not very often.

    It is true you can do better not using the battery by coasting, and get mpg more up toward 100 mpg but I do not find that practical or possible where I drive.

    If you try to run on the gas engine all the time you are guauranteed less mpg for other cases.
  9. kDB

    kDB New Member

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    the battery system itself is not the reason the prius works. yes, it is one of the main parts, but there is pleny of other stuff there. theres the ICE, very efficient, and it turns off when not needed. there's also the low drag coefficent...

    the fact is, the electricity in the battery comes from somewhere, and unless your plugging the car in and getting it from the grid, have a solar setup, ect., it's coming from the ICE. sure there's the regenerative braking, but something has to speed you up first so you can use that. EV for speed... your back to needing a source of electricity, and eventually in this loop you'll run out due to friction of the road and inefficient electricity transfer to and from the parts in the system.
  10. tomdeimos

    tomdeimos New Member

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    You're correct there are other efficiency improvements in the Prius. But the battery is a major one at low speeds.

    You can't get the super mpgs by using the engine. I have tried at various speeds. I always get better mpg on battery driving on flat roads at 40 mph than on engine at 43 mph. My best mpg is always when I am on battery about 2/3 or more of the time and I can drive at a steady speed.

    Any time the engine is going full time I generally get mpg's that are always below 70 mpg. This includes slow driving in winter too, when it is too cold for the engine to stop.
  11. jayman

    jayman Senior Member

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    Last summer, there was a lot of road construction on the highway that runs north of the Trans Canada to my hobby farm. A lot of 70 km/h speed limits.

    With the car at 70 km/h - about 42 MPH - the MFD claims a consistent 3.7 l/100km: 76 MPG Imperial gallon
  12. FreshAirGuy

    FreshAirGuy New Member

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    Someone else has already asked. Nonehtless are you referring the MPG shown on the display while driving or the MPG derived by dividing total miles by number of gallons used? Since the original inquiry referred to effciency which is most commonly an outcome variable rather than a process variable, the changing MPG is not a good measure of efficency. Others have already pointed out that gliding displays 99 MPG but that has little or nothing to do with actual number of gallons of fuel consumed.
  13. tomdeimos

    tomdeimos New Member

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    What I refer to as my mileage is my total trip mileage. I reset the display regularly except when I am checking over many tanks to check the mpg accuracy. (I'm doing that again now, to see if the code update changed anything.)

    So it includes my trip, usually to work via back routes that are quite repeatable. 15 miles or so total. I never measure going home, because traffic then is just too variable.

    Instant readouts of 80 plus are easy to get and mean nothing, as does the 99 when on battery. But battery use pays well when you are on gas only 1/2 or less of the time and are still getting 50 or better. I go by the total result.

    And now that I have my code upgrade I am on battery even more, and my mpg has gone up noticably as a result, except in the cold weather where the engine just has to run full time anyway to keep warm.

    For mpg 30 mpg is better than 40 due to air drag alone. And under 40 you will be on battery anyway if your car is working right. Only way to avoid it is in the country with pulse and glide but not practical around where I am.

    I am quite happy running on battery trying for 75 mpg or better instead of pulse and gliding and trying not to charge the battery to get 100 mpg with my speed varying all over the place.
  14. tochatihu

    tochatihu Senior Member

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    I have a copy of the US gallons/oF page from the spreadsheet, but could not find the entire spreadsheet, to give due credit to the author. Wayne Brown's program (HSD version) was used. I will try to atatch the image here.
  15. windstrings

    windstrings Certified Prius Breeder

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    Yea.. between those speeds you can utilize the "pulse and glide" technique that has shown to get up to 110mph?
    http://hybridcars.about.com/od/ownership/a/pulseandglide.htm

    Of course thats not too practicle for driving on the freeway! :lol:
  16. hobbit

    hobbit Senior Member

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    Seconded. Read up on what the Pittsburgh Marathoners did. It is
    nowhere near as simple as a sweet-spot speed -- you have to take into
    account what modes the system is running in. The idea with P&G is
    that you're running the engine efficiently, or not at all, and NOT
    cranking a lot of energy in and out of the battery but using your
    momentum to best advantage. For most people, above 41 mph and most
    bets are off WRT efficient engine use, unless you know how to use
    "warp stealth" that allows the engine to turn but not burn gas.
    .
    Wayne's site is http://privatenrg.com -- recommended reading indeed.
    .
    _H*
  17. vincent1449p

    vincent1449p Active Member

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    The author of the spreadsheet is Didier LUSUARDI and it was created using Wayne Brown's program.
  18. jdjeep98

    jdjeep98 New Member

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    :blink: Is it me, or does it seem odd to you, too, that you would get any reading below max (99.9) when you are using "battery only"... :blink:
  19. kDB

    kDB New Member

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    this happens when the ICE is running but is not helping move the car. usually when the ICE is warming up.
  20. jdjeep98

    jdjeep98 New Member

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    Actually, Energy Management is why the Prius works. The battery gets all its energy from the ICE, which burns gasoline. What makes the car efficient is not the conversion, which loses some of the energy in the process, but the management of the car's energy requirements.

    That management includes all the little weirdnesses like stopping the ICE when it's not required, regenerating electricity from the car's kinetic energy (something a gas-only vehicle cannot do ever), and balancing the use of electricity and gasoline. ALL the energy used by the car comes from gasoline. The battery is just a big (not big enough, in my opinion) energy buffer.

    Technically, you are correct, but it's like saying that the hammer is what makes a house. ;)
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