Best speed for best milage?

Discussion in 'General Motoring' started by Andrew Hess, Jan 19, 2004.

  1. Andrew Hess

    Andrew Hess Guest

    For whatever reason, I've always assumed that the best gas milage on a
    car was at about 75mph, or 3k rpm. I really have nothing to base this
    on, though. For the subies, in particular the my04 Forester 2.5x, what
    is the best mph for best mpg? I know there are probably lots of
    variables that enter into this (load, wind, etc), but in general, how
    fast?

    Andrew
     
    Andrew Hess, Jan 19, 2004
    #1
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  2. Andrew Hess

    TG Guest

    I have not done this on a Forrester but most consumer cars and trucks get
    the best at a much lower speed...55-65MPG. Let us know what you find if it
    doesn't bore you to death doing the experiment. TG
     
    TG, Jan 20, 2004
    #2
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  3. Andrew Hess

    Rodrigo Diaz Guest

    Warp Factor 7.
     
    Rodrigo Diaz, Jan 20, 2004
    #3
  4. Andrew Hess

    Bruce Hoult Guest

    It's hard for me to find out, since 75 mph is well over the speed limit
    here in NZ, but FWIW when I travel between Wellington and Auckland I
    have a choice of doing 80 - 105 km/h in heavy traffic on the wide and
    fairly straight SH1, or finding a hillier, more twisting, narrow
    tertiary road and taking a lot of corners at 80 km/h and doing 120+ km/h
    on the straights.

    I seem to get about the same MPG either way, or maybe even slightly
    better on the back roads.

    It's *very* noticable that hills that require a bootfull of gas in top
    gear (or even a downchange) at 100 km/h can be taken with only the
    lightest of pressure at 120+, so I can well imagine that 120+ might be
    more economical as well.

    Unfortunatly, with only a 4 speed auto, forcing it into 3rd all the time
    isn't really an answer, since you end up at 4000 RPM, not 3000, and that
    doesn't feel very economical at all (it would be 160 km/h in 4th).

    -- Bruce
     
    Bruce Hoult, Jan 20, 2004
    #4
  5. Andrew Hess

    Hawksoob Guest


    Best mileage would be zero. ;-)

    According to the cartalk guys, the speed you're going right after
    you've shifted into the highest gear is the most fuel efficient.
    Typically around 45mph. If you could conceivably drive that speed in
    that gear for the majority of the time, you would be getting the best
    mileage.
     
    Hawksoob, Jan 20, 2004
    #5
  6. Andrew Hess

    Bruce Hoult Guest

    No, that would be the worst possible, assuming that the engine is
    actually running.

    Sometimes I shift into top gear at 50 km/h, and sometimes at 150 km/h.
    Which should I pick?

    I probably shift into top gear most often at 109 km/h, that being the
    highest speed consistent with not getting a speeding ticket in a 100
    km/h zone. If not for that constraint, I'd probably shift at 120 km/h,
    in line with the previous poster's posited 75 mph.

    -- Bruce
     
    Bruce Hoult, Jan 20, 2004
    #6
  7. Andrew Hess

    Cam Penner Guest

    I've heard that the best economy on a four stroke internal
    combustion engine is when the engine is being lugged at low
    RPM. Lots of throttle, low RPM. This is the best
    efficiency for the ENGINE to make power, and has nothing to
    do with wind resistance. In theory, lugging the engine at
    low RPM in top gearing would produce the best MPG. The
    engine is producing the most power for the least gas, and
    running that power through the highest gear in the
    transmission.

    Unfortunately, lugging an engine at low RPM with full
    throttle is also supposed to be one of the hardest things
    you can do to the engine, so your real economy may vary.
     
    Cam Penner, Jan 20, 2004
    #7
  8. Andrew Hess

    Bruce Hoult Guest

    Cam Penner
    That's true, but only if the engine in this condition produces exactly
    the right amount of power to maintain speed on a flat road. For the
    vehicles we're talking about that is nowhere near being the case ... the
    engine will be producing far more power than required at that speed.
    Given the size of engines we have (e.g. 2.5L), and assuming an engine
    speed of 2000 RPM, I'd guess that a subie would be capable of pushing a
    gear giving around 160 km/h (100 mph) at 2000 rpm, which is twice as
    tall as normal gearing.

    That's true as well. You'd have to build it stronger than they are now.

    -- Bruce
     
    Bruce Hoult, Jan 20, 2004
    #8
  9. Andrew Hess

    Ross E. Guest

    I thought I read somewhere ages ago that when the engine is producing it's
    max torque it is at it's most efficient point in the rev range.
    Of course 3500-4000rpm in top gear would bring wind resistance into the
    equation so that wouldn't work.
     
    Ross E., Jan 20, 2004
    #9
  10. Andrew Hess

    Hawksoob Guest


    What that means is the fastest the wheels can go with the least amount
    of RPMs, which would typically be the slowest speed in the top gear.

    YMMV
     
    Hawksoob, Jan 20, 2004
    #10
  11. Andrew Hess

    Edward Hayes Guest

    Seems to be some misconceptions here. Maximum engine efficiency is where the
    engine is producing the maximum horse power per gallon of fuel consumed. The
    occurs at the maximum BMEP (break mean effective pressure). That usually
    occurs near the maximum torque value. The maximum fuel economy occurs where
    you get the most miles per gallon. With so many variables it's difficult to
    predict but I suspect it consides with the lowest vehicle
    speed(aerodynamics) and lowest engine speed where the engine is running
    clean and smooth. I have no data but, I am guessing that's about 16-1800 rpm
    and ~38 mph on my Forester S (fourth gear and torque converter locked up).
    eddie
     
    Edward Hayes, Jan 20, 2004
    #11
  12. Andrew Hess

    C. Brunner Guest

    Here's an EPA (U.S.) guide on fuel economy:
    http://www.fueleconomy.gov/feg/FEG2004intro.pdf

    There's a graph in that document that indicates the best mpg will be
    achieved at a speed of 45 to 55 mph. That's probably the standard
    government line, though, not taking all the variables into account.
    You can also get the standard EPA mpg figures for U.S. cars of various
    makes, models, and years at that site:

    http://www.fueleconomy.gov/feg/findacar.htm

    C. Brunner
     
    C. Brunner, Jan 20, 2004
    #12
  13. Andrew Hess

    Mickey Guest

    Edward is correct with his comments but the biggest factor has all but not
    mentioned and that's wind resistance. From an engineering standpoint, air drag
    is a ^3 (cube) function. What that means is it takes 8x power to double the
    speed not 2x as some might think. Without a lot of tech work it is hard to say
    exactly what speed will provide the best mileage but keeping it under 50-55 mph
    would be a good bet.

    Mickey
     
    Mickey, Jan 20, 2004
    #13
  14. Andrew Hess

    Edward Hayes Guest

    Mickey: I used the word Aerodynamics instead of wind resistance so we're on
    the same wavelength. eddie
     
    Edward Hayes, Jan 20, 2004
    #14
  15. From a "not an engineer" standpoint, I'd always thought it was a square
    (^2) factor, but regardless, wind resistance is THE killer factor.
    Back in the early '70s when the US instituted the 55 mph national speed
    limit in response to gas "shortages," I was led to believe that speed
    was chosen as the best "average" top economical speed for the "most"
    vehicles. Naturally, some were more or less affected by increases in
    speed: I drove a Fiat 850 Spyder at the time that got virtually
    identical economy between 55 and 70 mph, but others I knew saw widely
    differing results depending on what they were driving. I got a laugh at
    the time out of the highway patrolmen (several) who chased me in their 6
    or 8 mpg Dodge cruisers to give me tickets for "wasting" gas by going
    over 55 mph--while I was getting right at 35 mpg out of my Fiat! Oh,
    well...

    Anyway, in response to the OP's question, only testing will tell for
    sure! But as others have suggested, a range of 50-60 mph is probably a
    good target zone.

    Rick
     
    Rick Courtright, Jan 20, 2004
    #15
  16. Andrew Hess

    Bruce Hoult Guest

    Drag force is proportional to the square of the speed. Power required
    is proportional to the cube of the speed.


    Right. But it's completely dependent on design.

    Land barges in the 70's probably *did* do best at 55 mph, but there's
    nothing magical about that number. A Boeing 747 has its best fuel
    consumption (best range on a tankfull) at something like 500 mph.
    Different designs in terms of aerodynamics and amount of power available
    could produce a most economical speed anywhere in between (or outside)
    those numbers. As a bicyclist, my best range is probably at around 10 -
    12 mph.

    There is no scientific reason whatsoever to doubt that certain modern
    cars might well get their best fuel economy at 120 km/h (75 mph) or
    higher.

    -- Bruce
     
    Bruce Hoult, Jan 21, 2004
    #16
  17. Thank you! I knew there was a square factor in there somewhere...
    'Tis very possible... when you look at drag coefficients on cars
    today--regular passenger cars--which are not far off what race car
    designers were shooting for not that many years ago, the game has
    changed considerably. Plus gearing, engine output parameters, fuel
    delivery, tires, etc. can all be manipulated to produce a variety of
    results hardly dreamed of 20 or 30 years ago. Still, only hands on
    testing can tell us what a particular car can or will do, though. And
    even that's at least somewhat dependent on the driver: I've driven cross
    country in informal tests with other drivers, all going as close to the
    same speed as possible during their shifts at the wheel, where fuel
    economy varied by over 10% from one driver to another. So I might be
    able to get the same economy at 70 mph that another driver could only
    get at 60, which makes it difficult to say what's best for a given car!
    I'm thinking one would have to run many thousands of miles at speeds
    ranging from perhaps as low as 45 mph to 75 or better to establish the
    ranges and make a valid determination based on the combination of both
    car and driver. As they say, YMMV!

    Rick
     
    Rick Courtright, Jan 21, 2004
    #17
  18. Andrew Hess

    Edward Hayes Guest

    Many years ago when I worked for Argonne labs electric vehicle program we
    used a coast-down test to determine the horse power required vs. a vehicle
    speed. You take the vehicle to the highest speed and decouple the engine
    (declutch) and measure time vs. vehicle speed to establish a "coast-down
    curve. I cant remember the books name but I think it was "Vehicle dynamics:
    by Torboric. Wish I could find a copy as so much good stuff like comparing
    AWD vs. 2 wheel drive etc. Anyone familiar with this book?? eddie
     
    Edward Hayes, Jan 21, 2004
    #18
  19. Andrew Hess

    Guest Guest

    : Many years ago when I worked for Argonne labs electric vehicle program we
    : used a coast-down test to determine the horse power required vs. a vehicle
    : speed. You take the vehicle to the highest speed and decouple the engine
    : (declutch) and measure time vs. vehicle speed to establish a "coast-down
    : curve. I cant remember the books name but I think it was "Vehicle dynamics:
    : by Torboric. Wish I could find a copy as so much good stuff like comparing
    : AWD vs. 2 wheel drive etc. Anyone familiar with this book?? eddie

    I got my Masters degree working on a hybrid electric vehicle, and we did the same
    coast-down test. Bottom line is (as mentioned earlier) drag force goes with the square,
    so drag power goes as the cube. The rolling resistance force stays roughly constant with
    speed so for a given weight the power is linear with speed. Add those two, and you get
    something dominated by a linear term at low speed and cubic term at high speed. In older
    cars (>10 years old), these two values are equal at around 45 mph. In newer cars, it's
    more like 55-60 mph. That doesn't mean that's where it's most efficient, but rather where
    it goes much less efficient quickly.

    Given that, ignoring the engine size/gear ratio choices for a specific car, the
    best mileage is at the slowest speed. Problem is engines/gear ratios are generally sized
    for giggle-factor, not economy.

    -Cory

    --
    *************************************************************************
    * The prime directive of Linux: *
    * - learn what you don't know, *
    * - teach what you do. *
    * (Just my 20 USm$) *
    *************************************************************************
     
    Guest, Jan 21, 2004
    #19
  20. Andrew Hess

    Jim D Guest

    By experience in my 2000 Subaru Outback, with the 4-cylinder boxer engine,
    my best gas mileage occurs when I'm driving between 45 and 70 MPH. Over 70
    and it drops off noticeably. Under 45, and I'm usually in stop-and-go
    traffic, which gives so-so gas mileage. I just changed jobs, and my commute
    went from 22 miles down to 8 miles, one way. My miles-per-gallon also went
    down, from 25 to 20, cause the old commute was on Interstate highway and the
    new commute is in town.
     
    Jim D, Jan 21, 2004
    #20
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