My Mrs has a Focus Mk2 1.8 petrol, which has lived an urban life for the last 2,500 miles or so. Av MPG showing on the computer is 33.9 whereas my carefully monitored calculations show 37. Has anyone else noticed this kind of disparity?
Incidentally, we previously had a mark 1 focus with the 1.8 Zetec engine and this returned about 31/32 in the same circumstances.
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I believe the computer fuel consumption is always showing somehow less or more than we calculate. It's not only a Focus case. I believe a Vectra 1.9 CDTi 150HP is the most accurate in showing the fuel consumption.
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If, for example, the computer takes the average of all the (recent) instantaneous consumption figures will it be the same as a manually calculated long-term average anyway? I think we need a fuel computer expert/mathematician to solve this one.
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My Veccy's computer mpg is always wildly optimistic compared to the real world (i.e. brimmed tanks and Excel). I'd say it's overestimating by around 10%.
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If I recall NC's explanation correctly, the trip computer works out the average mpg by summing the amount of fuel it thinks it has injected, and then dividing by the distance calculated on the odometer.
There will always be a slight discrepancy between the fuel that it intended to inject and reality, and the odometer is not 100% accurate either, so over a decent distance an error is built up. It's still pretty good, given that a typical 4 pot may be injecting fuel 2000 times a minute..
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The Scenic's computer is accurate over a single tankful to around 1%.
VAG computers seem to be the most optimistic. The computers on both my in laws Golf TDI and a friend's Fabia VRS consistently show mpg figures 10-15% higher than reality.
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My Vectra CDTI is fairly accurate, matches my own calculations usually to within an mpg or two.
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IIRC the Ford trip MPG function is a rolling averge over the previous 100 miles or so, therefore will not match the calculated consumption though does reflect changes in driving style, road conditions etc, perhaps see how the handbook describes the function.
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I've sort of given up on mpg or kpl calculations. I now work out pence per mile by relating miles on the trip since last fill up to the cost of refilling. I am pleased to say all my cars seem to be doing a little better than they were. Not as good as I would like but better.
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My cost per mile calculations are being helped in no small part by Unleaded dropping from 108.9p/ltr on my last tankful, to 97.9p/ltr on the tankful I will purchase on the way home this evening.
One good thing to come out of the global economic traumas, even if it is only temporary.
Cheers
DP
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