How do I work it out?
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Stuart
How do you determine the most efficient speed to drive your car at on a motorway.
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Stuart - 2008-02-24 10:03:53 |
| Re: How do you determine the most efficient speed to drive your car at on a motorway. | Colin - 2008-02-27 09:51:52 |
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I don't know is the short answer, but I guess you also need to include the coefficient of drag in the equation somewhere as wind resistance varies with vehicle shape and speed. -- Quisling |
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| Re: How do you determine the most efficient speed to drive your car at on a motorway. | muymalestado - 2008-02-27 15:03:29 |
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Yes, drag - and, rolling resistance Vs speed, internal engine resistance Vs RPM, aspiration efficiency Vs engine temp or Vs RPM or Vs load, fuel quality from one month to the next, time since last full service. . . .
I suggest a big box of sandwiches, drive 1000km at constant speed (motorway only, obviously), fill up noting fuel added and repeat several times at different steady speeds - till sandwiches run out or cash runs out. Let us all know the result! |
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| Re: How do you determine the most efficient speed to drive your car at on a motorway. | Stuart - 2008-02-27 15:28:33 |
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Thanks muymalestado, would it matter what type of sandwiches I took?
"My Rover with a 1.8t engine is doing 2200 rpm at 55mph in 5th gear. Rising to about 3000 rpm at 75mph, and I imagine 3750 rpm at 90mph! " Lets have a look at this a different way. By dividing the speed by the rpm we should get an idea of efficiency (assuming each engine revolution uses the same amount of petrol).
The results are: So it appears linear. So why does every one say that efficiency decreases dramatically with speed?
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| Re: How do you determine the most efficient speed to drive your car at on a motorway. | muymalestado - 2008-02-27 15:55:47 |
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Well; I do not believe this will turn out linear.
The engine efficiency will not be; rolling resistance will not be; wind resistance certainly is not; ... I suspect university labs have done these tests and spent great time and money doing so just because the result comes from multifactoral parameters. Car companies will definitely know this data but would never release it. This does not invalidate the question - I recall it is best to get up to speed as fast as possible. But is that right? Drag racers would then be the most efficient - at least at startoff!
An old VW Polo or Golf had a vacuum at the carb, somehow, linked to a dashboard dial showing "MPG" at each moment. You don't see these now; why? |
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| Re: How do you determine the most efficient speed to drive your car at on a motorway. | Colin - 2008-02-28 21:20:27 |
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I think efficiency decreases with speed because drag increases in a non-linear way (I think it's a square relationship?). As the vehicle speeds up the air, instead of flowing round the slow moving vehicle, starts to compress in front of it - becoming denser and creating more resistance. I'm not an expert though! -- Quisling |
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| Re: How do you determine the most efficient speed to drive your car at on a motorway. | Simon Ridout - 2008-03-01 09:27:26 |
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My car, a diesel Golf Plus, has a fuel consumption display, both current mpg and journey mpg. On motorways, 50-60mph is the most efficient, with the mpg going down steeply over 70 mph. Constant speed, by using the cruise control is not the most efficient, slowing down slightly when going up hill and speeding up when going down is more economical for a given average speed. The resistance, both rolling and wind increase expodentially with increasing speed. -- Simonridout |
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