Optimum engine RPM

Discussion in 'Start Your Journey Here' started by Niall, Dec 19, 2016.

  1. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Yeah, that's running about 1200 rpm in 5th at 25 mph. Definitely on the low side, but plenty to maintain cruise on a flat road, which I was doing. Around town I usually keep it at 1500+, so 5th doesn't come until 30 mph! I REALLY want a 6th gear!
     
    phoebeisis and BillLin like this.
  2. davem

    davem New Member

    Some basic physics here that contridicts all of the above:
    1) air resistance is a cube function of vehicle speed so twice as fast will have 8 times the drag
    2) Spark ignited/gasoline/stoichiometric/ not diesel engines produce the most efficient horsepower at full open throttle at the lowest rpm that meets the required horsepower. This is due to lower pumping losses of the open throttle, lower vacum pull on the pistons. Also rpms increase friction and the energy to pump oil.
    3) touching your brakes means you put to much energy in earlier and wasted gas and brake pads.

    Summary: to maximze mpg drive as slow as possible, in the highest gear possible and dont touch your brakes (plan ahead).
     
  3. S Keith

    S Keith Well-Known Member

    1) square not cube. 2X speed = 4X drag.
    2) scroll up to post 6
    3) agreed, though this impact is diminished with regenerative breaking.
     
    RedylC94 likes this.
  4. RedylC94

    RedylC94 Well-Known Member

    You're confusing drag force with the power required to overcome it. The former is a square function of speed (as S. Keith said), while the latter is indeed a cube function. In your 2× speed example, 8 times the power for half as long is only 4 times the energy.
     

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