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Thread: How accurate is your ave MPG display on BC?

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    Regular Member renodci's Avatar
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    Default How accurate is your ave MPG display on BC?

    My BC says I'm averaging 44.9mpg (3.0cdti Auto); but having just filled up and calculated the 'real' figure, it works out at only 37.2! I know these things are never going to be 100% accurate, but 20% optimistic makes it a bit useless.

    I used to have a 1.9cdti that showed an average during my ownership of 51.1mpg - I wonder what that was really doing!

    Still; 37.2 out of a 3.0 auto, driven fairly enthusiastically is still pretty good - just not as good as it was leading me to believe.

    Is it typical for these to over-read by 20%? As far as I know my car is stock, so the reading shouldn't be skewed by a remap.

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    Regular Member rich r's Avatar
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    It's not that accurate, but then neither is any vehicle's fuel consumption computer, for a variety of reasons. It's difficult to get an accurate measure of speed because the distance travelled for each wheel rotation depends on tyre pressure, wear and the road surface friction. It's difficult to know how much fuel is used because all you can measure is the time each injector is open, but the actual amount of fuel will vary depending on fuel pressure and temperature at that moment in time. And a whole host of other reasons. It's even worse if you've got a Dual Fuel (ie LPG) model like mine, as the amount of LPG consumed is affected by even more factors, and the trip computer only measures the petrol injector signals (which aren't actually injecting anything).

    I tend to leave the display on trip average consumption, and just concentrate on getting the number as high as possible rather than worrying too much about the number itself. I do find mine tends to be within 10% though over a few hundred miles.

    Reset everything (including the range) next time you fill up and see if it's any more accurate.

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    Regular Member Dan_BlackSRi's Avatar
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    Vehicle : Vectra SRi

    Trim : XP1

    Engine : 1.9 CDTi 150

    Year : 2007

    Mileage : 66000

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    Over period I have had my latest SRi (only 3K miles) I compare the BC with brim figures and they have always been within 2mpg of each other.

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    Regular Member iain.condliffe's Avatar
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    Mine is currently under reading by approximately 10%

    quite surprised really so on average 43mpg mainly round town and 6 mike commute

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    Regular Member Penfold101's Avatar
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    Go into the options and make sure you're in UK Gallons and not US Gallons...

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    Regular Member Das's Avatar
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    Vehicle : Honda Civic

    Trim : ES with Honda Image and City Packs

    Engine : 1.8 iVtec

    Year : 2009

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    Mine is pretty good. Ive the biggest differences I have seen on mine are +/- 1.5mpg.

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    Vehicle : Insignia Grand Sport

    Trim : SRI VX-LINE NAV

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    I find the trip computer to be reasonably accurate, and certainly better than having to manage without it..

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    Regular Member renodci's Avatar
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    Quote Originally Posted by Penfold101 View Post
    Go into the options and make sure you're in UK Gallons and not US Gallons...
    Lol.......just checked this - and it was correctly set to UK. Just out of interest, I changed it to US Gallons and the Average Display changed from 44.9mpg to 37.4mpg.......within a whisker of the 37.2 it's really doing! I think I'll leave it set on US, as that gives a much truer reflection of what the car's really doing. Having Googled the size of a US Gallon v a UK gallon; it's 3.79 and 4.546 litres respectively. If my schoolboy maths is correct, that means the MPG on US Gallons should be about 20% less than on UK Gallons (because the gallon is 20% smaller). That would surely mean that changing it from UK to US Gallons, the average MPG display should INCREASE not decrease? It looks like the settings on my car are displayed the wrong way round? Does that make the car worth more because it's unique Lol!

    My car has got the larger 18" wheels (225/45 tyres) - presumably the rolling circumference of them is much the same as the factory-fit 17's?

    EDIT: Lol.......just realised that of course the MPG displayed should be less in US Gallons...........still strange how when set to US Gallons it shows exactly what I'm really getting on UK Gallons!
    Last edited by renodci; 19th October 2012 at 14:28. Reason: Brain now working!

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    Regular Member Penfold101's Avatar
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    Quote Originally Posted by renodci View Post
    Lol.......just checked this - and it was correctly set to UK. Just out of interest, I changed it to US Gallons and the Average Display changed from 44.9mpg to 37.4mpg.......within a whisker of the 37.2 it's really doing! I think I'll leave it set on US, as that gives a much truer reflection of what the car's really doing. Having Googled the size of a US Gallon v a UK gallon; it's 3.79 and 4.546 litres respectively. If my schoolboy maths is correct, that means the MPG on US Gallons should be about 20% less than on UK Gallons (because the gallon is 20% smaller). That would surely mean that changing it from UK to US Gallons, the average MPG display should DECREASE not increase? It looks like the settings on my car are displayed the wrong way round? Does that make the car worth more because it's unique Lol!

    My car has got the larger 18" wheels (225/45 tyres) - presumably the rolling circumference of them is much the same as the factory-fit 17's?
    I didn't bother to think about it all that much - I just know it's a common thing to check when the reading isn't what you expect..

    The tyres should have near the same circumference, but you can put the numbers into a tyre size calculator and find out. There are various settings in the computer for the different tyre size groups, so it is possible that it wasn't set to the right one wit the 18" alloys but how much of a difference it would make I don't know...

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    Quote Originally Posted by renodci View Post
    My BC says I'm averaging 44.9mpg (3.0cdti Auto); but having just filled up and calculated the 'real' figure, it works out at only 37.2! I know these things are never going to be 100% accurate, but 20% optimistic makes it a bit useless.

    I used to have a 1.9cdti that showed an average during my ownership of 51.1mpg - I wonder what that was really doing!

    Still; 37.2 out of a 3.0 auto, driven fairly enthusiastically is still pretty good - just not as good as it was leading me to believe.

    Is it typical for these to over-read by 20%? As far as I know my car is stock, so the reading shouldn't be skewed by a remap.
    Mine is total opposite BC state 37ish actual is around 43ish

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