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Thread: A chip the gets best fuel economy at say 70-75 mph?

  1. #1
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    Default A chip the gets best fuel economy at say 70-75 mph?

    I'm just throwing this out there, i don't know if its possible or what the performance ramifications would be, so i defer to the experts (calling Mark D).
    Just thinking that w/ the rising prices, it could be interesting if its feasible and nice to have it a higher cruising speed. just a thought...

  2. #2
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    There is something that has bothered me for a number of years. When I was in high school, I remember being told by my physics teacher "A car gets its best mileage at 45 MPH".

    Quotes like that were thrown around in the 70's as justification for the 55 MPH speed limit.

    Recently, I heard some so-called expert saying the same thing (only @ 55MPH) on a talk-radio show.

    They are all full of bull-sh*t. It was a generalization made perhaps 40-years ago when US car's had 3-speed automatics, weighted 2 tons, had engines that redlines at 4500 PRM and the aerodynamic properties of a brick.

    That generalization has not been true for many decades now, and it is a generalization - the constant speed at which a particular vehicle gets its highest mileage varies quite a bit.

    Anyway,

    From extended cross-country road-tripping, I've found that my 1990 535i (5-speed, Dinan Chip) gets its best mileage around 75 MPH, about 22.5 MPG according to the OBC.

    For most modern cars, with low CD's, efficient, high-revving engines, and 5 or 6 speed transmissions, I would bet that they get their highest fuel economy over 55 MPH.

    Results are specific to that car, but in my case, the speed limits should be raised, not lowered like so many people are saying.



    Quote Originally Posted by Jason
    I'm just throwing this out there, i don't know if its possible or what the performance ramifications would be, so i defer to the experts (calling Mark D).
    Just thinking that w/ the rising prices, it could be interesting if its feasible and nice to have it a higher cruising speed. just a thought...
    -Matt P
    '99 540iT

  3. #3
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    Default calculated from full tanks of gas.............

    i get 26mpg highway at around 80mph in my 540i/a.
    i also noticed in my 325e i got much better efficiency at 75-80mph.
    it all has to do with the gearing.
    my 540i/a is not geared to well for around town & therefore i suffer with 12mpg. i have a theory that if i change my diff. from 2.93 to 3.46 or 3.64 my around town driving would become more efficient due to the engine not needing to work as hard, but i would suffer on the highway.
    does this make sense?
    tim s.

  4. #4
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    Default Actually they aren't full of it, they were telling you correctly ,

    aerodynamic drag increases as a squared function with velocity.. the faster you go the more horsepower it requires and the worse mileage you'll get. Now modern day cars may be geared to get better mileage at 75 than in the past but they will still get better mileage at 55. If you look at some of the hybrid sites where they are really stretching for gas mileage this is clearly illustrated , where 10 mph difference will make a big difference for them in mileage. since 10% of 60 mpg is 6 versus 10% of 18 is only 1.8 , the point is much more clearly illustrated with hybrids... and they have the lowest cd going.





    Quote Originally Posted by Matt P
    There is something that has bothered me for a number of years. When I was in high school, I remember being told by my physics teacher "A car gets its best mileage at 45 MPH".

    Quotes like that were thrown around in the 70's as justification for the 55 MPH speed limit.

    Recently, I heard some so-called expert saying the same thing (only @ 55MPH) on a talk-radio show.

    They are all full of bull-sh*t. It was a generalization made perhaps 40-years ago when US car's had 3-speed automatics, weighted 2 tons, had engines that redlines at 4500 PRM and the aerodynamic properties of a brick.

    That generalization has not been true for many decades now, and it is a generalization - the constant speed at which a particular vehicle gets its highest mileage varies quite a bit.

    Anyway,

    From extended cross-country road-tripping, I've found that my 1990 535i (5-speed, Dinan Chip) gets its best mileage around 75 MPH, about 22.5 MPG according to the OBC.

    For most modern cars, with low CD's, efficient, high-revving engines, and 5 or 6 speed transmissions, I would bet that they get their highest fuel economy over 55 MPH.

    Results are specific to that car, but in my case, the speed limits should be raised, not lowered like so many people are saying.

  5. #5
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    Default Dangit Bill, you beat me to it, but I have a longer explanation!

    Your overall efficiency goes down (exponentially) the faster you go due to air drag – that one is pretty obvious. The faster you travel, the higher the drag force, the more force is required to keep you going a certain speed.

    Now, your engine has an efficiency curve, I would draw it but don’t have the time (I’m supposed to be working). It will have RPM on the x axis and efficiency on the Y and looks something like a bell curve (less efficient at idle and peak RPM, more somewhere in the middle) and will have multiple curves for the different load percentages (inertia of the car felt by the engine). Now, if you take your “drag efficiency” and multiply it by the engine efficiency, you will have your overall efficiency, which is what you want to maximize.

    If you fix your speed (say 70mph), your drag loss is a constant (depending only on the drag coefficient and speed), the only thing you can play with is the engine operating point, which is controlled through the gearing. At a certain speed, the closer you can get the engine to its BEP (best efficiency point (point in this case is RPM and load)), the better your overall fuel economy (at that speed). This is the whole point of hybrids – to use the electric motor at idle (no engine) and charge the batteries at the BEP (all engine).

    This however does not give the absolute best efficiency. That comes from fixing the engine at its best efficiency point (say X,XXX rpm and XX%load) and gearing to give the lowest drag force. The problem with that is is that if you change the gearing to give you a different speed and drag force, your engine load changes. If you try to figure that one out in your head (ie, which diff should I put in?), you’ll really get wrapped around the axle (pun intended) in a “which came first, the chicken or the egg?” type problem. The only way to solve it is to fix the gearing (low), run it through the whole range of RPMs, find the BEP of that system. Increase gearing, go through the whole process again. Increase gearing again, and again through the whole range of RPMs. Go through all that data and find the most efficient out of all that. That’s what the engineers do at car companies to determine the best gearing for your car, its not just “gear low for highway, gear high for city”.

    Now, if the engineer that designed the car said “75% of the time, the customers using the car are going 55mph”, so I’ll use the first method and design for that speed. Guess what? 55mph is the most efficient speed, period. If the engineer uses the second method, it could be anything, but I guarantee you that the car was not designed to be most efficient at 75 mph (at least the US models) because we’re not technically allowed to go that fast. Why on earth would you design for a BEP at a speed you can’t attain? If you’re building a total highway cruiser (and they’re not) the best you can hope for is to design for efficiency at 65, 70 tops. In reality, engines, transmissions, and drag have not changed drastically since the 70s. Engine efficiency is based upon completeness of combustion, intake temperatures and exhaust temperatures. While engines may be able to rev higher now than in the 70s, that doesn’t make the general operation of them any different, and they will still have similar efficiency curves based on RPM and load, albeit somewhat better, and therefore the you will still get your best gas mileage at lower rpms in high gear, usually near 55mph.
    93 525i / 01 330Ci / 98 Camry / 91 Volvo 240 / 99 Jeep GC

  6. #6
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    Default Actually the hybrids like the prius use a cvt transmission which runs the

    engine at the rpm which produces maximum efficiency almost all the time , thats one of the reasons they seem weird to people if because if you give it alot of gas the engine runs at a constant rpm and the gearing changes accordingly as you accelerate, you don't get the sensation you get as each gear shifts on a regular automatic. And typically the torque peak rpms is also the point of highest efficiency. Say around 4500 or so for the prius , thats the rpm it usually runs at constantly as your accelerating.




    Quote Originally Posted by DanDombrowski

    This however does not give the absolute best efficiency. That comes from fixing the engine at its best efficiency point (say X,XXX rpm and XX%load) and gearing to give the lowest drag force. The problem with that is is that if you change the gearing to give you a different speed and drag force, your engine load changes. If you try to figure that one out in your head (ie, which diff should I put in?), you’ll really get wrapped around the axle (pun intended) in a “which came first, the chicken or the egg?” type problem. The only way to solve it is to fix the gearing (low), run it through the whole range of RPMs, find the BEP of that system. Increase gearing, go through the whole process again. Increase gearing again, and again through the whole range of RPMs. Go through all that data and find the most efficient out of all that. That’s what the engineers do at car companies to determine the best gearing for your car, its not just “gear low for highway, gear high for city”.

    Now, if the engineer that designed the car said “75% of the time, the customers using the car are going 55mph”, so I’ll use the first method and design for that speed. Guess what? 55mph is the most efficient speed, period. If the engineer uses the second method, it could be anything, but I guarantee you that the car was not designed to be most efficient at 75 mph (at least the US models) because we’re not technically allowed to go that fast. Why on earth would you design for a BEP at a speed you can’t attain? If you’re building a total highway cruiser (and they’re not) the best you can hope for is to design for efficiency at 65, 70 tops. In reality, engines, transmissions, and drag have not changed drastically since the 70s. Engine efficiency is based upon completeness of combustion, intake temperatures and exhaust temperatures. While engines may be able to rev higher now than in the 70s, that doesn’t make the general operation of them any different, and they will still have similar efficiency curves based on RPM and load, albeit somewhat better, and therefore the you will still get your best gas mileage at lower rpms in high gear, usually near 55mph.

  7. #7
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    Default

    nifty!

  8. #8
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    Default

    I was thinking of the civic hybrid (which I got a $50 gift card to test drive, BTW), which alternates constantly between charging and assisting with the electric motor. That was all the technical engineering description they could give me about it (it flips back and forth), but when I noticed that it was always charging at 3500 rpm and always assisting at higher than that and at idle, I assumed thats how it worked.

    When I asked about how the toyota hybrid worked, all he said was that Honda's technology was better, with nothing to back it up. In his defense, I don't think he was particularly prepared to sell the hybrid, all he wanted to do was put me in a 4 cylinder automatic accord (I cringe at the dullness of the thought) like the other millions of zombies each year that buy that car.

    As far as the peak torque being the point of highest efficiency, that makes sense because at peak torque you can run the highest gear ratio and have your best shot at overcoming drag. I heard that somewhere else, but again, they didn't back it up with any references so I put it in the back of my head and forgot.
    93 525i / 01 330Ci / 98 Camry / 91 Volvo 240 / 99 Jeep GC

  9. #9
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    Default

    ive concluded i can do about 26mpg, avereging 75-80mph, on a level interstate i can maintain 80 at the 25mpg mark

  10. #10
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    Default

    Interesting. Thanks for the education.
    -Matt P
    '99 540iT

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