Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Best speed for fuel economy.

  1. Jul 7, 2010 #1

    what is the exact best speed for fuel economy?

    why is 5mph less economic than 75mph?
  2. jcsd
  3. Jul 7, 2010 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    33.7 mph is best
  4. Jul 7, 2010 #3

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    seriously, it is god that you asked this question .. we need a little more information ..it is like you ask..How high is the sky?
    whats the best speed for mpg economy for a bus?
    automobile? train. plane? truck...
    in the desert on dirt roads or highway with unlimited sped limits..in restrictive epa mandated smog dictatorship like California or wide open spaces where no emission equipment is required..like Daytona ??
  5. Jul 7, 2010 #4


    User Avatar

    Staff: Mentor

    For a car the most efficient constant speed is the lowest speed it can comfortably hold in its highest gear.
  6. Jul 7, 2010 #5
    As for me using 1NZ-FE VVT-i, u pumped about 15 liters (approx 4 gallons) I can travelling about 250 km (155 miles) at speed 100 kmh (60 mph). Actually, it depends on your driving style too.
  7. Jul 7, 2010 #6
    I agree with Russ. The internal combustion engine is most efficient at ~35% of redline and 80% of maximum torque, as shown in BSFC (brake specific fuel consumption). See thumbnail. For lower power requirements, the engine RPM should be reduced by staying in the highest gear to maintain high torque.

    [added] Note in the thumbnail that the lines of constant power in the BSFC map are hyperbolas. If you shift gears without changing velocity, you move along the hyperbola.

    Bob S

    Attached Files:

    Last edited: Jul 7, 2010
  8. Jul 7, 2010 #7
    At 5mph the engine is using more energy to simply keep itself spinning than it is to move the car.

    Other than that, it depends on many factors. If you go too slow, your engine will not be at its most efficient point. As has been mentioned, an engine is most efficient at a particular load and a particular rpm.

    Engines are also most efficient at wide-open-throttle. Just that fact itself means you have to be going at a decent speed.

    At the other end of the spectrum, the faster you go, the more air resistance (it goes up with your speed squared), so the more energy gets wasted due to friction.

    All these factors and more determine which speed is best for fuel economy. It will be different for every car.
  9. Jul 10, 2010 #8
    thanks for all of your help, i greatly appreciate it :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook