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Thread: Wasting gasoline in high gear

  1. #1
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    Wasting gasoline in high gear

    In manual cars for economy it's always good to keep the RPMs as low as possible by keeping the car in the highest gear possible with the car still moving and not stalling.
    If I had a 6 speed manual and I was going 40Mph in 6th gear but I would have to keep foot all the way down on the gas pedal to keep car moving in 6th gear at 40Mph, woudn't that waste gas because I'm keeping the gas pedal down as far as it will go to keep the car moving?

    Like on some old cars when the gas pedal is pushed that sends more gas to the engine. Since the engine cant go faster because the car is in 6th gear would the excess gas going in the engine be wasted?

    Or say that I go from a dead stop in 3rd gear and to go in 3rd I have to push the gas pedal all the way down to make the car move. Wouldn't that waste gas?
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  2. #2
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    Quote Originally Posted by QBridge
    In manual cars for economy it's always good to keep the RPMs as low as possible by keeping the car in the highest gear possible with the car still moving and not stalling.
    If I had a 6 speed manual and I was going 40Mph in 6th gear but I would have to keep foot all the way down on the gas pedal to keep car moving in 6th gear at 40Mph, woudn't that waste gas because I'm keeping the gas pedal down as far as it will go to keep the car moving?

    Like on some old cars when the gas pedal is pushed that sends more gas to the engine. Since the engine cant go faster because the car is in 6th gear would the excess gas going in the engine be wasted?

    Or say that I go from a dead stop in 3rd gear and to go in 3rd I have to push the gas pedal all the way down to make the car move. Wouldn't that waste gas?
    two replies: When the car goes 40 in top you just have push the throttle far enough to make that happen. Another thing is when you have to accelerate you may have to shift down to allow quicker up-revving of the engine. Actually the most economic way of accelaration is brisk almost full throttle periods in each gear. Moving the throttle slowly in each gear is less efficient.

    When you want to get going from a standstill in third gear, it is more a matter of fiddling with the clutch at low revs than giving full throttle with lots of slippage in the clutch as you suggest. Anyway this practice is to be avoided always....
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  3. #3
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    I disagree with that, the faster you accelerate, the more gas you would burn. The most fuel efficent way to drive a manual car would be to accelerate slowly and to shift when the next gear would put the RPMs just above stalling.
    If you had to design the most fuel efficent car possible (ignoring hybrid systems) you would make a car that at fuel throttle would produce just enough power for high way cruising (maybe 10 or 15 hp). This is because the partially opened throttle produces alot of intake drag, and maintaining the vaccum behind the throttle also produces inefficenty. Of course a car with only 15 hp would take years to hit 60 mph.

  4. #4
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    Well, I think that the best way of lowering the fuel consumption is to keep the engine revvs as close to the maximum torque as possible.

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    Quote Originally Posted by SLoppYJeeP
    I disagree with that, the faster you accelerate, the more gas you would burn. The most fuel efficent way to drive a manual car would be to accelerate slowly and to shift when the next gear would put the RPMs just above stalling.
    we have a (government sponsered) campaign in Holland called: "new driving" (literal translation, the meaning is really a new style of driving). Apart from the obvious things like shutting off your engine for an open bridge, or letting the car roll out (in gear) when you will have to stop anyway (for a traffic light), they also introduced the system of accelerating that I described earlier. I have to admit I was sceptical too, but it seems to work, and there was a technical explanation that I wish I could no reproduce, but I cant'
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  6. #6
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    Quote Originally Posted by jaromull
    Well, I think that the best way of lowering the fuel consumption is to keep the engine revvs as close to the maximum torque as possible.
    Well I am not so sure about that. For example when I drive 65 kph in my car I can do that in top gear at about 1450 revs, and in fourth it would be about 1800, which is closer to the 1750 at which I have max torque, but it needs a larger throttle opening.
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  7. #7
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    why can't we just drive normally
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  8. #8
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    Quote Originally Posted by coolieman1220
    why can't we just drive normally
    here's to you
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  9. #9
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    wait so in around 65km/h should i get it to top gear or what then?
    badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger badger

  10. #10
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    Quote Originally Posted by d-quik
    wait so in around 65km/h should i get it to top gear or what then?
    depends on your car. I was talking about mine, a low revving diesel, which runs at 30 mph at 1000 revs in top.
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  11. #11
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    Quote Originally Posted by henk4
    Well I am not so sure about that. For example when I drive 65 kph in my car I can do that in top gear at about 1450 revs, and in fourth it would be about 1800, which is closer to the 1750 at which I have max torque, but it needs a larger throttle opening.
    OK Hank, you are right, of course. It is not so simple, it depends even on the road profile, how much of torque and which rear is ideally suited for a smooth ride. Anyway the best torque/gear/revvs combination means the better use of the kinetic energy of each part of the car in motion starting with the pistons and ending with the weight of the car itself, i.e. you can avoid as much of changes in the throttle as posible. Of course I must admit it works especially for more simple fuel injection systems and carburettor-equipped engines. The advanced fuel injection systems have their own means how to lower fuel consumption and the influence of a "heavy foot" stepping on the accelerator pedal is not so pronounced.

  12. #12
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    Quote Originally Posted by jaromull
    OK Hank, you are right, of course. It is not so simple, it depends even on the road profile, how much of torque and which rear is ideally suited for a smooth ride. Anyway the best torque/gear/revvs combination means the better use of the kinetic energy of each part of the car in motion starting with the pistons and ending with the weight of the car itself, i.e. you can avoid as much of changes in the throttle as posible. Of course I must admit it works especially for more simple fuel injection systems and carburettor-equipped engines. The advanced fuel injection systems have their own means how to lower fuel consumption and the influence of a "heavy foot" stepping on the accelerator pedal is not so pronounced.
    so true about modern electronics to prevent the consequences of the heavy foot. And talking about kinetic energy, the best way to save fuel is not to brake , but to anticipate when you really have to slow down, and keep on speed in corners when it is possible, accelerating is the main cause of excess fuel consumption
    "I find the whole business of religion profoundly interesting, but it does mystify me that otherwise intelligent people take it seriously." Douglas Adams

  13. #13
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    Well, I DO NOT brake unless I MUST brake.

  14. #14
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    Here is my understanding:

    1. The maximum torque of the engine corresponds to the maximum efficiency of the engine, because it is the greatest force per revolution, equivalent to "per intake event," equivalent to "per unit of gasoline;" however this assumes that the air:fuel ratio is constant across rpms for full throttle, which is not correct, but might be in the lower rpm range.

    2. Full throttle should give the maximum efficiency because it gets rid of the obstruction of the throttle plate.

    HOWEVER: Full throttle is not necessarily the most efficient because the fuel mixture is probably not stoichiometric there.

    All of this is, as henk4 mentioned, gearing dependent. But, I believe the maximum efficiency would be at: full throttle, at the engine's maximum torque, going as slow as possible.

    Probably the easiest way to observe this would be to see how a car with a CVT and electronic throttle control behaves.

    EDIT: I wrote this after reading through post #6

  15. #15
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    Quote Originally Posted by henk4
    we have a (government sponsered) campaign in Holland called: "new driving" (literal translation, the meaning is really a new style of driving). Apart from the obvious things like shutting off your engine for an open bridge, or letting the car roll out (in gear) when you will have to stop anyway (for a traffic light), they also introduced the system of accelerating that I described earlier. I have to admit I was sceptical too, but it seems to work, and there was a technical explanation that I wish I could no reproduce, but I cant'
    For years I too have read & heard those views from fuel economy experts that reasonably decisive acceleration from standstill to the nominal cruising speed will decrease consumption, vs the eggshell-on-pedal method. This applys both to carby and EFI. One reason cited is that because the car gets up to speed quicker it thus spends more time & covers more distance in fuel-saving high gear, instead of prolonging lower-gear useage by dribbling up to speed

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