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Low on gas question: which to choose?

  1. Aug 3, 2009 #1
    I've been pondering this questions for a while.

    If you are driving and you are low on fuel and you need to get to a certain destination, is it better to slam on the accelerator, using up more fuel but getting to your destination quicker...or is it better to driver slower, using less fuel but taking longer to get to your destination? Which method would use the remaining fuel to its maximum efficiency to get you to where you need to go without having the engine die on you?

    Thanks.
     
  2. jcsd
  3. Aug 3, 2009 #2
  4. Aug 3, 2009 #3
    You use far more fuel by going quickly, so using any revs at all is a poor idea.

    Most efficient use of fuel consumption occurs at cruise, so typically (50ish mph) top gear full open throttle (full load) and 30% revs.

    Driving syle also aids fuel consimption, smooth acceleration and braking and use of overrun will decrease fuel consimption.
     
  5. Aug 3, 2009 #4
    As fast as you can go at low RPM will give you the best distance.
     
  6. Aug 3, 2009 #5

    Ben Niehoff

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    The article discusses the energy cost of maintaining velocity, but you also must consider the energy cost of acceleration, which is much more significant if you are driving in city traffic or any area where you must stop at many traffic lights. You want to avoid having to slam on your brakes, because that wastes all the energy it took to get yourself up to speed. Therefore (on city streets), you should drive at a speed that is not so high that you have to brake suddenly, and you should accelerate only moderately when getting a green light, etc. Often the ideal speed on city streets is (gasp!) the speed limit.

    On the highway though, yes, every mile per hour above 60 is probably lowering your fuel efficiency by some quadratic function. Unless you have the opportunity to coast a lot. I've gotten some pretty good gas mileage going 80-85 with a fully loaded car (in the 35-38 mpg range), but my car is rather small, and the road was very flat.
     
  7. Aug 3, 2009 #6

    Office_Shredder

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    I think it's still more gas efficient to just stay at 60 rather than accelerate to 80 and then coast down to 60. If there's a slight downslope to the road that may change. Maybe accelerating from 30 to 50 and coasting would be better, but accelerating past 55ish is never going to help
     
  8. Aug 3, 2009 #7
    Well first off you clearly want to be going a speed that uses the most efficient gear ratio. After that point, increasing the RPM's will continue to increase MPG by making the fuel used by the alternator for powering electrical components (such as lights, battery recharge, AC, radio, and computer) proportionally less compared to the fuel used for moving the vehicle.

    You waste fuel every time you use your breaks, but let's just assume we're talking about an open road. The energy required to increase speed grows exponentially as you approach the speed of light, but that effect is completely negligible for automobile speeds...although a similar effect occurs due to drag forces, and I think this is the primary factor the limits your MPG at high speeds. Obviously that would depend a lot on the aerodynamics of your vehicle.

    Personally, I feel like my most efficient speed is about 75 (on flat terrain).
     
  9. Aug 4, 2009 #8
    I'm pretty sure I saw this tested on braniac, and the slower (and less aggressive) you drive, the better. Instead of relying on them though, let's think about it...

    You burn a lot of fuel in accelerating, as you have to overcome the inertia of the vehicle to bring it up to speed. Generally, your engine (burning fuel) is generating your propulsive force and F=ma. So more acceleration (or mass) means more force required by the engine, means more fuel burned. So, a constant speed is probably better than slowing down and speeding up.

    Drag is proportional to the square of the velocity of the considered object. The force resisting your car (drag and rolling resistance) the most is drag. The faster you go the larger this force gets, in a squared fashion. So, high velocities are out too.

    Your engine has a power curve for different gears (and a fuel consumption curve). there must be an optimal speed/rpm value where fuel consumption is least. And the best way to value your fuel consumption would then be in l/hour or gallons/hour. You could cross reference this value with the accompanying speed (km/h or mph) to find which speed and fuel consumption combination will get you the furthest.

    I'd say:
    -accelerate slowly and change gears smmothly
    -keep a constant speed in the highest gear where the car sounds "comfortable" ie. not in very high revs.
    -don't brake unnecessarily
    -turn the aircon (uses a lot of fuel to run the compressor) off and close the windows (may slightly reduce drag).
    -you could slipstream behind a truck (reducing your apparent drag force), but this is illegal in many countries.
    -use a gps to find the most econmical route to your destination (but preferably the nearest petrol station)
     
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