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Thread: Cruise Control - uses more fuel?

  1. #1
    Regular Member Doug-SRidirect's Avatar
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    Default Cruise Control - uses more fuel?

    Just a thought, but Iím certain my Vectra uses more fuel when the cruise control is used extensively.
    My theory: With no cruise control, on motorways you tend to speedup going downhill and slowdown going uphill, witness the positioning of speed cameras often towards to bottom of long hills. In a subconscious effort we are throttling back going up-hills and letting the car gather its own momentum going down-hill.
    With me so far?
    Well, using cruise control, the throttle opens as much as necessary to maintain the constant speed uphill (on my auto, it even changes down a gear sometimes) and to make matters worse, it closes the throttle completely going downhill in an attempt to prevent the carís speed from rising Ė thus wasting all that potential energy. Youíve gained no speed going downhill and as soon as you hit the bottom and start ascending, the throttle opens, and it powers you for the full length of the hill.
    Maybe my imagination but for the last couple of weeks Iíve not used the CC on my M1 commute and think Iím getting a couple of extra mpg overall. More likely itís my imagination, but itís a thought nonetheless.
    This is just a theory and Iím not so tight as to let it change my driving style, Iíll be back using the CC next weekÖ.and hang the expense!!!
    Any thoughts anyone?
    Doug

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    Regular Member Ozz's Avatar
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    Default

    I have been thinking the same for a while now. To confirm one way or the other I reset the trip computer thingy & covered a certain amount of Km's using then not using the CC. I made sure the car was carrying the same amount of added weight & fuel. At the end of it all my fuel consumption dropped using the CC as oppossed to not using it

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    Regular Member Harry's Avatar
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    Default

    i've been wondering the exact same thing, i've not tested whether or not this is true but my theory is as follows:

    doug -> i mainly agree with you, except for the bit about travelling down. i don't think you are wasting the momentum of the car because the engine will cutout unless it is required to accelerate and therefore your trip computer screen should read 999,99 instantaneous MPG, hence increasing the MPG figure.

    One thing i don't understand is why the instantaneous MPG reading is greater when the car is in gear but with no load on the engine i.e. decelerating or driving down hill, as opposed to when the car is in neutral and coasting, where the instantaneous MPG will show approximately 70 MPG and vary drastically. Anyone got a clue?

    I think overall the CC will reduce the MPG returned though.

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    Default

    I'm at a loss how you can use your CC on the M1, its all slow, stop, slow, stop.

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    Default

    CC is useless on inclines and descent as you are right - it doesnt foresee the advantage of carrying momentum through, also on twisty roads it can make the car unsettled as it decides to load power on or off mid bend, living in the flatland - it is fine and usable most of the time though. I use the CC like a throttle and will click it on and off all the time, off downhill descent - and then back on uphill (thats why my 3.0 CDTi gets over 50mpg!)

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    Regular Member Doug-SRidirect's Avatar
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    Default

    I start work at six am!

    On the overrun the fuel is cut off totally but when it's ticking over the tick-over jet (or whatever) opens and it uses more fuel!

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    Regular Member Harry's Avatar
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    kitchpoo ->50mpg!!! i'm gobsmacked!

    have you installed a diagnostix chip or similar? were you still getting such good mpg before the power upgrade?

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    before the pbox it was about 50 , now its about 51, a small increase.

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    Regular Member Tim2100's Avatar
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    Quote Originally Posted by hmha0
    One thing i don't understand is why the instantaneous MPG reading is greater when the car is in gear but with no load on the engine i.e. decelerating or driving down hill, as opposed to when the car is in neutral and coasting, where the instantaneous MPG will show approximately 70 MPG and vary drastically. Anyone got a clue?
    Yes the car will use zero fuel when in gear but no load as the moment of the car will keep the engine spinning, modern engines need no fuel to keep turning, whereas if you are coasting out of gear it will have to use a small amount of fuel to keep the engine going on tick over.

    Tim.

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    Regular Member Harry's Avatar
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    Default

    i see...thanks for clearing that up for me Tim.

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