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Optimum speed for mileage per gallon

  1. Dec 9, 2007 #1
    I imagine that this magnitude is dependent on a lot of factors, but does anybody know a good source to get the relationship betwen speed and mpg for at leas families of cars?
     
  2. jcsd
  3. Dec 9, 2007 #2
    Go to CarTalk.com and look at December, 1995.
     
  4. Dec 10, 2007 #3

    russ_watters

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    The optimum speed (with no air conditioner running) is generally the lowest speed that you can easily maintain in your highest gear.
     
  5. Dec 11, 2007 #4

    rbj

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    what exactly do you mean by "maintain", Russ? i don't think "lugging an engine" is the most efficient use of it. even on perfectly flat and smooth pavement.
     
  6. Dec 11, 2007 #5
    when the gearbox doesnt make weird sounds, i guess russ meant this
     
  7. Dec 11, 2007 #6

    rcgldr

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    I remember this same belief from the 1970's, but it wasn't true. For cars back then the optimum speed was around 45mph. This is because gasoline engines are very inefficient at producing small amounts of power, and running at higher speeds, with more of a power load, results in better milage, because the increase in rate of fuel consumed is less than the increase in the rate of speed.

    Modern cars have fairly tall gearing in the form of overdrive, and better aerodynamics than cars of the 1970's, so they should be more efficient at higher speeds. Considering that EPA measures highway milage at 55mph (this has just changed), the car designers may have designed their cars to get the best milage at 55mph.

    So other than a 1300cc VW bug from the 1970's, or a hybrid, the optimum speed for most cars will be between 45mph and 55mph.
     
  8. Dec 11, 2007 #7

    Integral

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    Which is about the slowest you can comfortably go in the highest gear.
     
  9. Dec 11, 2007 #8

    russ_watters

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    That's where the "easily" part comes in.
     
  10. Dec 11, 2007 #9

    russ_watters

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    I drive stick, but don't most automatics only have 4 gears? Also, gear ratios vary a lot by engine and type of car. Ie, an SUV will be geared a lot lower (or at least be able to handle a lower rpm). So I think that's too narrow a range - maybe 35-55.
     
    Last edited: Dec 11, 2007
  11. Dec 11, 2007 #10

    stewartcs

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    My automatic has 6 gears.
     
  12. Dec 11, 2007 #11
    So, there's an easy way to do this. Some cars have a built-in mileage indicator. Rent one and drive it in all different ways and watch the indication. When I did this, I found I got the best mileage at about 1500 RPM in the highest gear. This was on level ground (such as we have in western PA) and corresponded to about 40-55 mph. Presumably this would vary a bit in different cars, but that answer seems to "ball-park" it.
     
  13. Dec 11, 2007 #12

    RonL

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    For my first post i'll give a firsthand experience.
    Last year i purchased a used F350 truck, with a 351 V-8, the transmission is a 5 speed ( the final gear is overdrive ).

    Two trips out of town, with speeds of about 60 MPH, produced a fuel average of 11.25 MPG, my big supprise came when i started a part time job, about 13 miles from home, and traveling thru town in stop and go traffic ( top speed between 35-40 MPH ) average speed while moving is about 30 MPH, produced a fuel average increase to about 12.5 MPG.
    My thoughts are less wind resistance, and lower average speeds, actually produced an increase in MPG, where i least expected it.

    In line with what Russ said.

    If someone will point me to the proper place, i have a few thoughts and questions, about how to add a power system ( electric ) to this truck, that will provide 13 miles each way. The current power system will remain intact for heavy or long haul use.

    Thanks
    RonL
     
  14. Dec 11, 2007 #13
    Then, best mileage will be around the rpm's which give the highest torque with the highest gear. Does it make sense?
     
  15. Dec 12, 2007 #14
    no...
     
  16. Dec 12, 2007 #15
    No? Just no?

    I imagine that regime for highest torque is likely to be the most efficient (power vs fuel), and using the highest gear will produce the lowest number of explosions for a given mileage.
    Am I wrong?
     
  17. Dec 12, 2007 #16

    rcgldr

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    The amount of power generated for the amount of fuel consumed is best, but because of aerodynamic drag, it's consuming more fuel per mile traveled.
     
  18. Jan 2, 2008 #17
    from what i have read, wind resistance is not linear. meaning that wind resistance at 60 mph is not 2x the resistance at 30mph, it is lower. although the engine is most efficent at it highest torque rpm, the increase in air resistance at this speed and in top gear more than compensates to make lower mph driving more efficent.
     
  19. Jan 2, 2008 #18

    DaveC426913

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    This chart was done a few years ago on my 1993 Chrysler Intrepid:

    http://www.davesbrain.ca/miscpix/mileage.gif

    What I find odd about it is:
    - the lack of curvature. I would have thought it would level out to the left.
    - the minimal effect of the AC. It has a smaller effect than changing highway speed by 20kph.
     
  20. Jan 2, 2008 #19

    DaveC426913

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    The formula is such that 2x velocity will result in 4x wind resistance.
     
  21. Jan 9, 2008 #20
    good to know daveC, i didnt know what the actuall formula was. thanks
     
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