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Thread: What is the most economical speed to drive at?

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    Regular Member sheepish's Avatar
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    Default What is the most economical speed to drive at?

    As the title says.

    I commute 72 miles a day on a motorway and with cuise control at 70mph I get about 42mpg out of a 1.8 petrol. I'm sure the most economical speed is lower but does anyone know what it is???

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    Ex-Staff Full Member John LE's Avatar
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    56mph is normally regarded as the most economic speed to drive at

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    Regular Member sheepish's Avatar
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    Mmmm, I know thats the speed that's always been quoted, but is that still the case with modern cars???

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    Regular Member taffy bhoy's Avatar
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    0mph

    dont know to be honest, 56mph seems to be the magic number, but cant see how with different sized engined cars, as well as all the different makes/models of cars on the market today, that 56mph can be correct?

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    I think they only say that because its the slowest speed where lorries dont have to overtake you. Unless lorries are limited to that for economy reasons.

    In every car i've ever had i seem to get the highest mpg in the slowest speed i can do in 5th without labouring the engine

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    Regular Member PANDY 3.2 V6's Avatar
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    I think they only say that because its the slowest speed where lorries dont have to overtake you. Unless lorries are limited to that for economy reasons.

    In every car i've ever had i seem to get the highest mpg in the slowest speed i can do in 5th without labouring the engine

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    Regular Member Grimy's Avatar
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    56mph is not an arbitrary figure. It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed. Assuming the only other resistance is from the tyres and that this is similar for all cars then the major force that the car has to overcome is the air resistance. Hence 56mph is the most fuel efficient speed ! No wonder then that car manufacturers publish data for their cars at 56mph as it will show them off at their best !

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    Regular Member Hideous's Avatar
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    It's not the same for all cars. I've seen some individual tests that concluded around 30mph was the most economical speed for that particular car - on a flat road.

    here's just one example I found with a quick search on google http://en.wikipedia.org/wiki/File:Fu...speed_1997.png
    Last edited by Hideous; 23rd November 2011 at 23:26.

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    Quote Originally Posted by Grimy View Post
    56mph is not an arbitrary figure. It corresponds to the speed at which the air flow around an object changes from laminar flow to turbulent flow. It always occurs at this speed, independent of the shape and size of the object and is therefore the same for all vehicles travelling through air. At this speed there is a sudden drop in the air resistance, a blip in the graph where the general trend is increasing resistance, or drag, with increasing speed. Assuming the only other resistance is from the tyres and that this is similar for all cars then the major force that the car has to overcome is the air resistance. Hence 56mph is the most fuel efficient speed ! No wonder then that car manufacturers publish data for their cars at 56mph as it will show them off at their best !
    56mph is a very very old government test speed as far a i know! The transitional airflow at 56mph is onei have never heard of........but 56mph is about as slow as you should ever go on a motorway.
    Anyway as Pandy says the most ecconomical speed by a massive margin is 1000 to 1500 rpm in top gear without labouring the engine.

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    Ex Vec-C Admin & Founder Graeme's Avatar
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    my vec 2.2 petrol auto was best on long run at 70 constant speedon a long drive. The corsa cdti i'm driving now seems better at 40-50mph though have yet to do a long run at higher speeds constantly. (oys of chuffin roadworks all the way to work)

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