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Thread: Trip Computer live MPG readout.

  1. #1
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    Vehicle : Vectra Elite


    Engine : 1.9 CDTI

    Year : 07

    Mileage : 127,000

    Default Trip Computer live MPG readout.

    Just got a 57 plate CDTI SRI 150 and for the first time ever, I have a trip computer with the live MPG readout. A guy at work tells me that they are not that accurate so I thought I would ask the question here. So, are they accurate? Also, the same guy says that cruise control uses more fuel. I find that the car in cruise control seems to show a better live MPG then I can achieve. Again, is this true?

    Thanks, and I hope I don't appear stupid.

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    Regular Member parapaul's Avatar
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    OK, in no particular order...

    The trip computer isn't that accurate, they generally seem to be about 10% optimistic (ie if you're getting 40mpg it will show 44 or 45) but they're good for seeing trends.

    In my experience CC doesn't make a huge difference, it accelerates more gently and more often than you probably would on your own, but I don't think it uses more.

  3. #3
    Regular Member chrisschild's Avatar
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    Vehicle : Vectra Hatchback

    Trim : SRi

    Engine : 2.2 DTi

    Year : 2003

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    With Cruise Control, I find the engine is being fed an almost constant flow of fuel, over a long journey (especially on the motorway) I find I can get the computer to read 5-6mpg more than on the Cruise Control.

    Though I must admit, there are times I think....I'll use it!

    As for the MPG read out, as said, I'm sure they will be out by around 10% as will be the speedometer (this accours on all cars).

    I'm using the road trip to germany as a fuel consumtion measure, i.e, reset the comp when I leave the fuel station at the start, and see what the reading is when I brim the tank after returning. Then I'll have a Computer MPG and Fuel Reciepts of Literage. By using a little math, I will be able to work out the % the computer is out by. (though this may vary from car to car & engine to engine!)

    Chris

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    Regular Member Penfold101's Avatar
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    At the end of the day, the car will use what it uses... Whatever the figure you may come out with at the end, it'll be expensive....

  5. #5
    Regular Member Steel's Avatar
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    Vehicle : Vectra

    Trim : Sri

    Engine : Z19DTH

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    There will always be an element of error in any fuel, speed or mileage calculation. The main thing to consider is tyre wear. A new tyre with perhaps 8mm of tread will have a greater rolling radius than a worn tyre with 2mm of tread. It might seem silly but with worn tyres the wheels are making more revolutions per mile on worn tyres than on new tyres, therefore you are using more fuel to cover the same distance, therefore more revs and more fuel. The computer can only measure the amount of fuel used to turn the engine at a certain number of revs so will have an element of inaccuracy. Similarly, even if you measure the amount of fuel used to cover a certain distance the same level of inaccuracy applies as the number of miles recorded on your odometer will be similarly affected.

    The only truely accurate way of measuring fuel economy is to measure the miles per gallon over a known distance with a known amount of fuel, so in my opinion the figure given by the fuel computer is as reasonably accurate as you can get in normal use.

    The cruise control also drives more economically than I do in any case as I don't seem to help myself from getting carried away, they just seem to want to go faster all the time

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    Vehicle : Vauxhall Astra-J

    Trim : Elite

    Engine : 2.0 CDTi

    Year : 2013

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    Quote Originally Posted by Steel View Post
    A new tyre with perhaps 8mm of tread will have a greater rolling radius than a worn tyre with 2mm of tread. It might seem silly but with worn tyres the wheels are making more revolutions per mile on worn tyres than on new tyres, therefore you are using more fuel to cover the same distance, therefore more revs and more fuel.
    That explains why my car (2.2 petrol auto) is now only doing approx 24 to 25 mpg (with 225/45/17 tyres fitted) whereas before it was doing approx 28 to 29 mpg (with 215/50/17 tyres fitted).

    Those figures, btw, are worked out by brimming the tank and getting the calculator out, but also compares with what the fuel computer says in the car as well.

  7. #7
    Regular Member Steel's Avatar
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    Vehicle : Vectra

    Trim : Sri

    Engine : Z19DTH

    Year : 2005

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    Quote Originally Posted by Big Dave View Post
    That explains why my car (2.2 petrol auto) is now only doing approx 24 to 25 mpg (with 225/45/17 tyres fitted) whereas before it was doing approx 28 to 29 mpg (with 215/50/17 tyres fitted).

    Those figures, btw, are worked out by brimming the tank and getting the calculator out, but also compares with what the fuel computer says in the car as well.
    Theoretically I would have thought that would only be the case if you have the speed sensor recalibrated as well, it will still think you are doing the same miles per revolution otherwise which would mean that although you are covering (marginally) less miles it would still be doing the same number of revs (I think) been a while since I did rev/rolling radius calculations.

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    Vehicle : Vectra Elite


    Engine : 1.9 CDTI

    Year : 07

    Mileage : 127,000

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    Thanks guys, guess I will have to do my homework.

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