Does engine RPM affect gas mileage?

  1. ShawnD

    ShawnD 986
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    It's commonly believed that running your car at a lower RPM (highest gear possible), will get better gas mileage. We hear this from our parents and friends, and we assume it's true because it seems to make sense, but has anybody ever done a study to prove this?

    I'm asking because I monitor my car's gas mileage very closely, and it seems to make absolutely no difference how I drive. The car will get the same mileage regardless of me driving fast or slow, high gear or low gear.
     
    Last edited: Apr 12, 2007
  2. jcsd
  3. Integral

    Integral 7,346
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    Assume that you travel a set distance at the same speed, first in 3rd gear, then in 4th. Can you see that the engine will turn more times in the lower gear. Each revolution of the engine will "consume" the same amount of air/fuel mixture. Therefore you must consume more air/fuel mixture at the lower gear.

    Now a lot of modern engines are getting smarter about feeding fuel so, the assumption of a constant a/f mixture may not be valid. With intelligent fuel metering the millage difference may be small.

    The biggest difference to fuel consumption is your rate of acceleration. If you like to feel some acceleration and take pride in your ability to get to 60mph (100kph) then you will see improvement by playing "old lady" for a while.

    You do more work when doing 5s to 60 vs 10s to 60. This increased work MUST be reflected in fuel consumption.
     
  4. I think it makes a difference, just calculate the difference in gas mileage a standard transmission vehicle gets when someone who drive one well drives it against someone who sucks at it.
     
  5. Danger

    Danger 9,879
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    There is a reason for low gears, though. 'Lugging' the engine by trying to use a higher gear than is called for is extremely wasteful and not good for the engine.
     
  6. Ivan Seeking

    Ivan Seeking 12,520
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    Even with intelligent fuel and air management, there will still be an optimum choice for any given speed and load. That is part of the idea behind hybrids - the engine can always be operated at peak efficiency by precisely controlling the generator load and engine RPM.
     
  7. Stingray

    Stingray 674
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    There are plots that can be looked up for a typical engine's torque output versus rpm and fuel consumption. It is typically better to keept the revs as low as possible, but it is certainly possible for fuel consumption to start getting worse before the engine starts lugging. Besides "direct" efficiency issues, you also have smaller oil and bearing losses at lower rpm.
     
  8. ShawnD

    ShawnD 986
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    That's not true at all. In third gear I can go 50km/h when barely touching the gas pedal. To maintain a speed of 50km/h in fifth gear, I need to floor it. lower rpm * more fuel per ignition = similar amount of fuel being burned.

    That's not true either. Kinetic energy is entirely determined by speed; time is not a factor.

    This seems entirely possible.
     
  9. It depends, not on rpm as much as the pulse width of the injectors (assuming efi). low rpm lugging = longer injection time, vs high rpm small injection time.

    Car and Driver did a study a few years ago and concluded that moderate acceleration (60-70%) and short shifting (couple thousand below redline) was optimal.

    For cruising pick a gear that allows you to keep your foot off the gas. You can track these with a laptop and make nice graphs.
     
  10. Integral

    Integral 7,346
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    Someone needs to retake (or may be take?) a physics course.

    Remember Newtons laws? F=ma?
    Now re read my post and make a bit of an effort to understand it.
     
  11. BobG

    BobG 2,346
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    Where you drive makes more of a difference.

    If you have to accelerate many times during your trip, you're going to get worse gas mileage. Toss in the fact that the main reason you're having to accelerate a lot is because you're spending a lot of time sitting still at a stop light with the engine running. In the city, being able to time the lights and avoid stops and starts will have more of an effect on your mpg than how fast you actually drive.

    If you drive on expressways or long highways, I don't think your speed matters a lot until you hit some high threshold, where upon your mpg stops to drop fairly quickly and that's directly related to the number of rpms the engine is running. That high threshold varies from car to car. I think most of the vehicles I've had can handle up to around 65 mph before they ever seem to start doing any real work, but that depends on the engine's capabilities vs the car's weight and aerodynamics.

    As to which gear, you're probably talking about the choice between two different adjacent gears. If you chose to drive around in 1st gear instead of 4th, you'd definitely get worse gas mileage, at least until the car overheated and died - after that, you'd save quite a bit of money in gas.

    Edit: I have noticed I get somewhere between an extra 1 and 2 mpg on my first couple of tanks when I'm headed East on vacation vs my mileage returning home. Dropping 4,000 feet in 600 miles is a lot better than gaining 4,000 feet in 600 miles. Plus, the prevailing winds tend to be towards the East and Jeeps have the aerodynamics of a brick.
     
    Last edited: Apr 13, 2007
  12. Integral

    Integral 7,346
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    Ok Shawn,

    I just returned from a short drive where I obsereved various conditions.

    I drive a '94 Ford Probe with a 5 speed manual trany and a 2.5L V6.

    @55mph

    3rd gear -> 4500RPM
    4th gear -> 3500RPM
    5th gear -> 2500RPM

    going from 3th to 4th I had to let off on the gas, to reduce the RMP, agian to get from 4th to 5th I had to let off on the gas.

    Now in 5th gear I am running near the low end of my power band, to accelerate I would drop back to 3rd or 4th and run the RPM up to the new desired speed, then shift back up to 5th to cruise.

    Can you see that it must take more gas to travel a mile in 3rd gear at 4500RPM then in 5th gear at 2500RPM?

    That is at constant speed, the real gas consumption occurs when you accelerate. The harder your acceleration the more gas you will use. That is simple physics.
     
  13. Moonbear

    Moonbear 12,266
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    Maybe it's time for Shawn to get a new transmission if he has to floor it in 5th gear to get to 50 km/h when he's barely touching the gas in 3rd, unless they number the gears in reverse order on Canadian cars.
     
  14. Kurdt

    Kurdt 4,941
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    Depends if he means hes flooring it from a standing start or from 2nd to 3rd or second to 5th. Then you would have to floor it because you'll be way out of the power band going 2nd to 5th. Once you hit 50 you can cruise in 5th.
     
  15. ShawnD

    ShawnD 986
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    In canada you go through the gears in the order 1-2-3-4-5. Do you start 5-4-3-2-1 in the US? 5th gear at 50km/h is about 1500RPM.
     
  16. An engine does NOT necessarily take more fuel when turning more revolutions in a given period of time. How far you have your foot into it is also NOT a direct indication of how much fuel you are sucking in a given period of time. RPM and throttle position together will usually determine pretty accurately how much fuel is going in. Engine vacuum is a fairly reliable (relative, not absolute) indicator of how much fuel is going into an engine.
     
    Last edited: Apr 13, 2007
  17. Moonbear

    Moonbear 12,266
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    No, lowest is 1st gear here too. That's why I was puzzled. If you're flooring it in 5th and not doing any better than that, it sounded like a transmission problem to me. But, kurdt's explanation that you might have meant putting it in 5th when it didn't belong in 5th made some sense as to what might be going on too. Of course, shifting too soon is going to do a lot of weird things both to mileage and the car, but if you're using your gears as you're supposed to, then Integral's explanation is the most sensible.
     
  18. Ivan Seeking

    Ivan Seeking 12,520
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    Cars are mostly designed to be most efficient in the highest gear while at normal highway speeds, but this characteristic is not intrinsically true of engines. See page three of this pdf.
    http://www.oznet.ksu.edu/library/ageng2/L886.PDF

    Note that the best fuel economy for a given power requirement does not always occur at the lowest engine speed. For an output of 50% of the maximum horsepower of the engine, the most efficient operating point is about 63% of the maximum RPM - at 102% of the minimum fuel consumption.

    The most efficient operating point [fuel consumption/Hp]min for the engine overall occurs at 58% of max power, and about 69% of max RPM. This would be the desired operating point for a hybrid's engine.

    Note also another motivation for hybrids - the terrible inefficiency of the engine as we approach the no load condition. I see this as a huge advantage for hybrid drivers in large cities. In bumper to bumper, stop and go freeway traffic, the fuel savings should be tremendous. So I suspect that some hybrid drivers will get a much greater advantage than might be expected.

    [Ironically, from an environmental engineering pov, it would make the most sense to allow standard vehicles to use diamond lanes, and make the hybrid drivers sit in traffic]
    Late edits
     
    Last edited: Apr 14, 2007
  19. Integral

    Integral 7,346
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    What did you say?
    RPMs is not an indicator, nor is throttle position.

    Except that throttle position and RPM are an indicator???

    Make up your mind!
     
  20. Wind resistance is enemy....

    Lowering your RPMs and speed will greatly increase efficiency and lower the drag force working against your vehicle. The effect is pretty dramatic in most cars above 60mph.
     
  21. Integral: What I mean is that without knowing what both throttle position and RPM are doing at the same time you cannot say what fuel economy is doing. In other words, you cannot just simply say (as you have) that more RPM guarantees more fuel consumption no matter what. There are more sides to this than just RPM.

    Edit: I said that throttle position is not a DIRECT indication and that an engine does not NECESSARILY take more fuel at higher RPM. You imply that I stated:
    I would read the above quote as stating that RPM and throttle position should never be considered at all.
     
    Last edited: Apr 14, 2007
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