Most fuel efficient speed to drive?

Discussion in 'General Motoring' started by mvl_groups_user, Aug 30, 2005.

  1. Back in the 80's I remember my dad telling me that the most fuel
    efficient speed to drive his car was at 45 mph, and that it was a spec
    that was published for many car models.

    With the gas price absurdity these days, I'm just curious if is there
    an equivalent most efficient speed for today's cars? (Specifically my
    05 EX-4 accord)

    mvl_groups_user, Aug 30, 2005
  2. mvl_groups_user

    Greg Guest

    Approx. 0 kph (0*0.6213= 0 mph) provided the engine is not running.
    Greg, Aug 30, 2005
  3. mvl_groups_user

    Elle Guest

    Coupla anecdotes to throw into the mill:

    My 1991 Civic seems to get its best fuel mileage somewhere between 35 and 55

    Someone recently posted here that going over 3000 RPM reduced mileage. That
    figure may vary somewhat, but I have noticed that cruising at 70 mph in 5th
    gear (of course) with my car puts me over 3000 rpm and reduces my fuel
    mileage. I do not get my best mileage on long highway trips, with the cruise
    control set at 68 mph.

    Avoid rapid accelerations.

    It seems staying in gear (instead of coasting with the engine at idle) and
    braking with the engine improves my Honda's fuel mileage, too, because
    (according to someone here) of the way the fuel control etc. system is set
    Elle, Aug 30, 2005
  4. mvl_groups_user

    flobert Guest

    For every carburated car i've owned, the optimal speed is around
    60mph. This is, i believe, why the US speed limits were set to 55 in
    the oil crisis.
    depends on the hill, and what you do with the coasted momentum really
    - use it to boost your vehicles speed, and you culd save fuel.

    Never really bothered to be honest, since fuels so cheap in the US.
    flobert, Aug 30, 2005
  5. mvl_groups_user

    B Squareman Guest

    Instead of speed watch the RPM. The sweet spot for most Hondas
    are from 2100 to 2400 rpm. Since some people may be driving on
    the wrong gear at 55-mph.

    The new VTEC-E has a different higher sweet spot. Check your
    information source.
    B Squareman, Aug 30, 2005
  6. mvl_groups_user

    B Squareman Guest

    That's because the fuel only cuts out (some carburetors too) when
    in gear and above 1180-rpm and gas pedal released (carburetor
    rpm is different). Idling is big business. Cruising is only a fraction of
    the injection pulse width if you measure it. Your best bet is to keep
    moving (not to slow, not too fast) in gear to increase your miles
    per gallon.
    B Squareman, Aug 30, 2005
  7. mvl_groups_user

    TeGGeR® Guest

    That was one reason. 55mph was determined in tests to be a good compromise
    between speed and mileage. The cops loved it too, once they discovered they
    could make megabucks from all those new scofflaws driving on roads designed
    for much higher speeds than 55. Hello, Fuzzbuster!

    The other reason was the new pellet-type catalytic converters that were
    mandated for 1975. It seems that they had a tendency to fire pellets out
    the tailpipe if exhaust pulses got too violent. No pellets = no conversion.
    Modern monobloc cats are immume to this.

    It sure is (thank god).

    I've also not found much difference in my mileage with car speed. Not
    enough to make me do anything about it, anyway.

    On average, I get about 27 mpg. A low of about 25 is seen at sustained
    sppeds over 90 mph, and a high of about 30 at sustained speeds of about
    60mph. I only conducted my speed tests once (60 is boring, 90 is too cop-
    friendly), so I can't confirm my results.
    TeGGeR®, Aug 30, 2005
  8. mvl_groups_user

    Elle Guest

    Dunno. My 91 Civic is fuel-injected.
    We disagree.
    Elle, Aug 30, 2005
  9. Wind resistance is another factor that has not been discussed so far.
    The relationship between wind resistance and speed is complex.
    However, at low speeds, the amount of wind resistance depends linearly
    on speed. For example, is the speed of the car is increased form 20
    mph to 40 mph, the wind resistance. will roughly double. However, at
    higher speeds, the wind resistance will begin to increase more
    sharply, so that doubling the speed may triple or quadruple the wind

    Part of the reason the spped was set to 55 was because with cars back
    then, that was the approximate speed at which wind resistance began to
    be an important factor. This became obvious if one listened to wind
    noise. At 45 or 50 mph in my 1976 station wagon, wind noise was barely
    noticeable. But at 75, conversation became difficult.

    Modern cars are much more aerodynamic than they were 30 years ago
    (except for those Chrysler products with huge, boxy, ugly grills), so
    the wind resistance probably does not become an important factor until
    a higher speed, but at some point it is going to take much of the
    available engine power to just keep the car moving.

    Elliot Richmond
    Freelance Science Writer and Editor
    Elliot Richmond, Aug 30, 2005
  10. mvl_groups_user

    Elle Guest

    Wind resistance (as measured by power required to overcome aerodynamic
    friction) should vary roughly in a square or cubic relationship with speed.
    I suspect what's really at work here are the conditions under which your
    typical car engine is designed to operate most efficiently. That the typical
    passenger car is not designed for optimal efficiency at 20 mph nor 75 mph
    makes sense, since the average driver's average cruising speed is probably
    closer to 35 to 60 mph.
    I wouldn't use the decibel level to indicate anything more than wind
    resistance goes up with speed according to a square or cubic relationship.

    (I lean towards cubic, from a units analysis standpoint, but I may be
    missing something from empirical studies on car drag.)
    All engine power is strictly to keep the car moving. "A body in motion tends
    to stay in motion, unless [the nasty F-phenomenon kicks in, which it will]."

    We can't eliminate engine (internals), wind, or rolling friction.
    Elle, Aug 30, 2005
  11. mvl_groups_user

    al Guest

    Agghhhh ... not this conversation again!? ;p

    Grrrrrrrrrr .....

    al, Aug 30, 2005
  12. mvl_groups_user

    al Guest

    Sounds rather low. I would have thought 55-65mph a sensible speed in 5th,
    though quite how anyone can set cruise control (or even use it for that
    matter!) at that low a speed on a motorway is quite beyond me!
    Give it a break! You guys are pretty much *given* petrol. At least this is
    a Honda group, so most of you aren't as selfish as your fellow countrymen in
    their gas-guzzlin' 4x4 pieces of crap. Try paying £60 to fill your tank
    each time (over $100).

    al, Aug 30, 2005
  13. mvl_groups_user

    DH Guest

    Not if it had poor aerodynamics, which would have been pretty common back
    then (especially if the car involved was not brand-new in the '80's but a
    car 5 years old, maybe dating from the mid-'70's, even). At low speeds,
    aerodynamic drag will be unimportant but it increases with the square of the
    DH, Aug 31, 2005
  14. mvl_groups_user

    Rich Guest

    The lowest speed without lugging the engine in the highest gear(5th). On
    level ground 45 to 50mph in 5th gear sounds about right(off the top of
    my head) and probably puts out about 2200 RPM. Anything below 1800 RPM
    starts to labor the engine. Above 65 mph much of your fuel is consumed
    pushing air out of the way of the car. All experts & mechanics(I am
    neither) out there, please correct me if I am mistaken.
    Rich, Sep 3, 2005
  15. mvl_groups_user

    Matt Ion Guest

    This question was printed in the latest Road & Track magazine... there's
    a very good, extensive answer there (check out the issue with the
    "flat-out one-mile" tests on the cover).
    Matt Ion, Sep 3, 2005
  16. mvl_groups_user

    Matt Ion Guest

    Actually, you can also find the R&T article here:
    Matt Ion, Sep 3, 2005
  17. mvl_groups_user

    gsl Guest

    if i drive 70-90km/hr in my 04 civic (automatic tranny) i usually get
    50-55 mpg consistantly
    gsl, Sep 3, 2005
  18. mvl_groups_user

    Doug McCrary Guest

    You drive in kmph and figure distance/fuel unit in mpg?
    Doug McCrary, Sep 4, 2005
  19. mvl_groups_user

    gsl Guest

    yeah, canadians sorta use a combination of the 2 systems. or at
    least i do. :)
    gsl, Sep 4, 2005
  20. I communte through many stop lights and stop signs each day. I've been
    looking for the most fuel efficient acceleration for my 05 Accord (4

    I've seen posts saying that cars in general drive most efficiently if
    driven near peak torque RPM, which for the accord is around 4500. My
    car's an auto, so I don't have too much control over RPM's. But the
    automatic likes to hover around 2000 RPMs, so the only way to be close
    to 4500 is almost flooring the starts.

    But I've also heard that the harder starts are less fuel efficient.
    Based on the Nav MPG, I think I've found keeping RPM's under 2000
    (sloooow acceleration) is more efficient.

    I don't mind speed of acceleration, since my commute is on 1-lane roads
    where no one can pass, and regardless of how fast I drive, I just get
    stuck behind another car at the next stop light.

    Anyone able to comment one way or the other on hard vs. soft

    mvl_groups_user, Sep 22, 2005
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