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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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