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Thread: Actual v Computer 8mpg difference (used to be 2mpg)...any ideas?

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    Default Actual v Computer 8mpg difference (used to be 2mpg)...any ideas?

    I've always kept track of my mpg figures when calculated at the pump and the car always seems to average about 40-42mpg per tank (about 500 miles). The on board computer always used to show about 1-2mpg better than this.

    I personally think that it should be slightly better than that with some of the runs I've done with a light foot, but still only get this average.

    Over the last few tank fulls the computer has been returning figures around 50mpg on long runs where I would have expected to get about 50mpg, and yet the actual figure has ben about 8mpg less.

    Is this massive difference normal? Or does it indicate that my economy isn't as good as it should be?

    Any ideas?

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    Regular Member DAVIDD88's Avatar
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    there never accurate just a rough indication mate i would not worry

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    If you reset the mpg and not the rest it can go a bit wonky, once told me i was averaging 79 mpg for about 30 miles then revised itself down to 45

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    One thing I have noticed....

    I usually buy fuel at Sainsbury's or Asda due to cheap fuel prices.

    The other week I decided to go to Shell and buy their Shell Fuelsave.. What a difference, I've got about 290 miles from HALF a tank opposed to 450 odd miles for a full tank, well impressed!

    Might be with a change if you are buying supermarket fuel!

    **I do not work for shell or am not an advocate of Shell, just my personal experience!**

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    Quote Originally Posted by mattyc View Post
    I've always kept track of my mpg figures when calculated at the pump and the car always seems to average about 40-42mpg per tank (about 500 miles). The on board computer always used to show about 1-2mpg better than this.

    I personally think that it should be slightly better than that with some of the runs I've done with a light foot, but still only get this average.

    Over the last few tank fulls the computer has been returning figures around 50mpg on long runs where I would have expected to get about 50mpg, and yet the actual figure has ben about 8mpg less.

    Is this massive difference normal? Or does it indicate that my economy isn't as good as it should be?

    Any ideas?
    Mine often tells me I'm doing 999.9 mpg going down hill with foot off the accelerator - if I could keep that up everywhere I go I would have to stop occasionally and syphon diesel out!

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    I always fill up at my local esso station and never use supermarket fuel. Also always reset all the settings on comp. I just find it a bit weird and I don't like not knowing why, just bugs me. Guess I'll iust have to learn to live with it unless their is either a way to improve actual economy to get closer to what the computer thinks it's doing, or re-calibrate the computer in some way?

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    The discrepancy could also result from the different ways being used to measure how much fuel you've used.

    The computer uses the fuel injector signals. The injectors have a fixed flow rate, and they're opened for a certain amount of time by the ECU to inject the volume of fuel it's calculated is needed for the amount of air entering the engine. So by looking at how long the injectors are open for, the computer can work out how much fuel has been injected into the engine in any given time period.

    You're using the forecourt fuel pump's counter as a guide to how much fuel has been put in. This is obviously a lot less accurate than the precise injector timings - but if you put a large volume you put in, the error isn't too bad.

    Mine's always way out - because it's a Dual Fuel. The trip computer works off the petrol injector signal. But there's no petrol being injected - the LPG system has it's own injectors which are open for a different time period than the petrol ones (the LPG ECU uses the petrol injector signal to work out how long to open the LPG ones for, but it's not the same period). So at the moment, my average consumption is showing as 41mpg - but I know it's more like 37mpg.

    What I find the trip computer most useful for is to put it so it's showing average consumption for the trip. Then just try to get the number to keep going up and not worry about what it actually is. That then gives you an indication of whether your current driving style is good or bad for economy. It's surprising how one short burst of acceleration can take several mpg off your average.

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    Yeah, I tend to do that too... It's sadly quite addictive and drives my wife nuts!

    Interesting about the involvement of the injectors. I've had all 4 replaced since I bout it. First 2, then another 2. Both seem to have co-incided with the comp getting less and less accurate. Is this a coincidence??

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