What is absolute best speed for highest mpg?

Discussion in 'Gen 3 Prius Main Forum' started by Robert Holt, Oct 19, 2013.

  1. Robert Holt

    Robert Holt Senior Member

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    Assume a flat road at sea level, winds are calm, no precipitation and standard temperature and barometric pressure. What is the optimal speed to get the absolute best mpg out of a Gen III Prius ?
    One poster mentioned 41 mph in passing but I have not seen any hard data and would very much like to know where the maximum efficiency point is if anyone has that info. Hard to find a stretch of road in my area where I could do this test well or I would do it myself, but maybe someone has already done so.
    (Airplanes come with maximum endurance and maximum range speeds listed and I really think the Prius ought to also.)
     
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  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Bob Wilson has done exactly that. (except he does not live right at sealevel, 600 feet may be a better guess)

    [​IMG]

    Near 60 MPH and near 75 MPH are relative high points.

    Updated MPG vs MPH chart | PriusChat
     
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  3. mad-dog-one

    mad-dog-one Prius Enthusiast

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    Zero MPH consumes no fuel if the ignition is off. ;)
     
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  4. David Beale

    David Beale Senior Member

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    My experience is around 70-80 km/hr. That's around 45-50 MPH.
     
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  5. Robert Holt

    Robert Holt Senior Member

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    The Bob Wilson graph suggests a max at 20- 25 mph, but your experience suggests 45-50 mph. Interesting. Did you happen to try lower speeds? I want to know for the case I must stretch the range as much as possible due to a lack of gas stations in the sparsely settled areas of Alaska and Canada.
     
  6. Mendel Leisk

    Mendel Leisk Sand Pounder

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    We've got two options for getting into town. One's via freeway with 90 kmh posted limit. The other via highway with 80 kmh limit, with 50 kmh stretches at both ends. The latter's our much preferred route now. Easier on the nerves too. ;)
     
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  7. dbcassidy

    dbcassidy Toyota Hybrid Nation, 8 Million Strong

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    How much distance is achieved?

    DBCassidy
     
  8. energyandair

    energyandair Active Member

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    Looks to me as though its the slower you go, the better the mpg, at least to somewhere below 25mph

    The 100mpg at both 25 and 19 mph looks like the maximum mpg readout on the display. If so, both may actually be better
     
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  9. fuzzy1

    fuzzy1 Senior Member

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    Bob's chart indicates only that the max happens somewhere below 25 mph.

    Some previous threads, and some equations from the Japanese hypermilers, point to a max MPG in the vicinity of 15 mph.
     
  10. David Beale

    David Beale Senior Member

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    Well, I've watched the mileage at very low speeds, below 50 km/hr (30 MPH) and it appears the consumption goes up. Of course, at those speeds to be safe, you'd have to be in the city in traffic or in housing areas trying to avoid being hit in intersections, and even then, around here you stand the chance of being run over! ;) I would NEVER try to extend the distance on a tank by going that much slower than traffic. You may drop consumption but you would run the risk of not making it due to being hit!

    As an example for my impression (and that's all it is as I've not done measurements, just watched the readout and results at the end of the trip), heading home from West Edmonton on the Yellowhead where the speed limit is 70 km/hr I have seen my best mileage in my so far short ownership of Pearl S. I've seen consumption for the 15-20 km trip as low as 3.1 l/100 km, though I would guess the average over many trips is probably in the 4.1 range.

    Remember, at really low speeds the engine will shut off and you will be running on the battery. This is NOT fuel efficient. If you watch the instantaneous consumption readout while driving in housing sub-divisions carefully to avoid intersection runners you will probably see what I see - consumption will reach a minimum around 60-70 km/hr and go up when you go very slow. This is probably because there is a "base consumption" of fuel vs time when the car is "on" and at very slow speeds you don't cover enough ground to get the distance number high enough to record good mileage.
     
  11. tumbleweed

    tumbleweed Senior Member

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    As David ^^ said, you must keep the ICE running for maximum mileage at a steady speed. Probably 41 or 42 MPH. On a non-hybrid it would probably be the slowest you could go in the top gear. Of course if there are hills, traffic, etc. things change and it becomes a judgment call.
     
  12. mediahound

    mediahound Active Member

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    Here's a good webpage to read from Tesla. The Model S has a similar coefficient of drag and they seem to recommend 55 MPH.

    Your Questions Answered | Tesla Motors

    It's true that the slower you go the better the mileage, but you get diminishing returns because slower becomes less practical and potentially dangerous on the freeway.
     
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  13. tumbleweed

    tumbleweed Senior Member

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    Does the Tesla offer a hybrid? I thought it was all electric. The 55 mph in the Tesla advertisement seems to indicate the speed for a typical range on the battery, the maximum range might be slower. What is being discussed here is the steady speed for maximum gas mileage in an HSD hybrid, quite a different thing.

    I agree too slow is dangerous on the freeway but that is really not relevant to the discussion. The OP ask what speed produced the best gas mileage under ideal conditions. No one is suggesting you drive 40 mph on the freeway.
     
  14. mediahound

    mediahound Active Member

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    Driving for maximum range in an EV should not be considered that different than driving for best MPG.
     
  15. tumbleweed

    tumbleweed Senior Member

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    I checked the Tesla advertisement again and changed my post accordingly. I did not see anywhere in that page that 55 mph produced maximum range. I think they simply wanted to give people some idea how far they might go at a sedate 55 mph highway speed. Maximum range might well be accomplished at a slower speed. Reading the graph they provide maximum range in the Tesla might be at about 20-25 mph.
     
  16. cary1952

    cary1952 Member

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    +1
     
  17. KA8

    KA8 Junior Member

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    Some people are way too obssessed about mileage. Yes mpg matters and mpg is the main reason for getting a prius but c'mon get a life and drive it like any other car. Hypermiling and all that is ridiculous, life is stresful as it is. When I'm on fwy I find myself going 70-75mph like aby other car and still getting 47 avg mpg
     
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  18. Robert Holt

    Robert Holt Senior Member

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    The scenario I have experienced is low fuel late at night in totally deserted areas on very rural roads with 0 traffic , so I could set the speed at any level. With manual shift Otto cycle I would know the peak of the torque curve of the engine and keep the tach at that point while in top gear, which often was 50 mph or so, but the Atkinson cycle Prius engine has a flat torque curve from very low rpms on up, so it seems like the slower speeds would be better as aerodynamic drag is less. I would prefer real world, standard condition test data to theoretical models as models may or may not accurately represent the entire real world situation. So thanks for your input, folks! But I am still unsure exactly what speed to set in the above situation.
     
  19. Mr.Electric

    Mr.Electric Member

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    I take a 25 mile long drive once a week in my gen 3. On the way at 2pm on the freeway in heavy traffic I average 35mph and get about 58-60mpg

    On the way back at 9:30pm with no traffic at 70 mph I get 44 - 45 mpg.
    I have non lrr tires that suck up a few mpgs too.
     
  20. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    [​IMG]
    Look at the grey curve in the above graph (for the Gen1) the shape of that curve will depend on your fixed load, if like me, you always run headlights it will be a higher MPH where the max MPG occurs.

    Gen 1 and Gen 2 should have a 'poor' MPG in the lower 40s. Gen 2 can often match Gen 3 near 60 MPH, the only speed where it comes close.
     
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