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Real fule economy figures

anparkinson

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Location
Hastings, East Sussex
Car
2007 Accord 2.0
My 2007 2 litre Tourer always seems to average about 36 MPG with a mixture of town, A-road and motorway driving according to the dash readout.

Taking the mileage between fill ups and calculating correctly (allowing for the fact that the odometer over-reads by 2.5%), the real figure is always around 33 MPG.

That's 9% out. I guess it's in their interest to make you think the cars are better on MPG than they actually are.
 
Yeah, never trust an onboard computer.
Always do the maths.
 
anparkinson said:
My 2007 2 litre Tourer always seems to average about 36 MPG with a mixture of town, A-road and motorway driving according to the dash readout.

Taking the mileage between fill ups and calculating correctly (allowing for the fact that the odometer over-reads by 2.5%), the real figure is always around 33 MPG.

That's 9% out. I guess it's in their interest to make you think the cars are better on MPG than they actually are.
To get the correct mpg, taking into account the odometer over-reading is absolutely the right thing to do.

But if you want to compare with the accuracy of the on-board computer, it doesn't know that the odometer over-reads LOL

So the proper comparison is 36 mpg on the computer, and 34 mpg based on true fuel usage, which makes the computer about 5% to 6% out. My guess for the inaccuracy is that the computer calculation is based on the integrated time of all the pulse widths at the injectors, and either that integration is inaccurate, or the fuel pressure is a tad higher than used in the calculation.
 
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