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Car MPG reading vs calculated reading - why the big discrepancy?
I recently purchased a 2015 Vauxhall Astra diesel 1.6 CDTi, attracted by the high quoted MPG of 76 (combined). I always calculate the cars MPG when I fill up, as well as recording the figure the car says. Having now done close to 10,000 I am surprised that my figures are always so far from what the car says, averaging 7.4mpg lower - both values are also much lower than the quoted MPG, with the car claiming 68.3 and my calculations yielding 60.8. I know that cars seldom reach the quoted MPG, but was wondering what would cause such a big difference between what the car thinks it is doing, and what it is actually doing, and whether this is common to all manufacturers/models?

Asked on 20 April 2017 by Alastair Penman

Answered by Honest John
The system in the car does not measure actual fuel used. It merely calculates it from a set of parameters, and the calculation is usually set to give figures that are 5 - 10 per cent better than reality. 60.8 is good for this engine. I'm only getting 48 from the same Renault engine in a Renault Kadjar 4WD.
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Real MPG enables you to register the fuel economy you are actually getting against your make and model of car, its engine and type of transmission. Every time you enter an average consumption figure against your car, the overall average automatically adjusts to reflect it.
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More spacious than previous Astra. Available with impressive 1.0 Ecotec petrol. Five star Euro NCAP rating. Good ride comfort. 17 inch wheels and 45 profile tyres give 1.6T outstanding grip.
 

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