the reason i said that the on board computers calculation of fuel economy is incorrect (lie)

is that from personal experience and as has been discussed before in this forum that is the case , it is highly optimistic in its calculations.

in my case the computer claims fuel economy of 8.1 litres per 100 (diesel) however if you do the calculations your self i get 9.1 litres per 100, or 9.4 per 100 when using the A/C.

i use my car for work and keep detailed records for tax purposes and can assure you that these figures are correct,

so again the onboard computer lies

I'm averaging 10.1 litres per 100km (onboard computer) with my 3.0 diesel.
I drive daily, mainly city (like 95%).

I think that's pretty impressive for almost 2,7tons heavy car.
Something must be wrong or you floor every light, haha.

While I accept your information on your own measurement difference, it isn't necessarily a widespread characteristic that the OBC reads out incorrectly. Mine has a maximum error of 0.2 l/100 km, based on filling it each time and measuring mileage. I have checked it on five BMW models that I have owned, and this has been typical. Maybe the diesel model has some additional variable.

There is always the possibility that your fuel pumps are not accurate, unless you use a different one each time and get the same delta.

obviously i cant be sure about fuel delivery pumps however i always fill it full and generally at the same cost, allowing in cents per litre variations

however what i really wanted to impress upon the OP was that you need to do the calculations your self and not rely on the obc for your determinations of economy

Okay I'm going to let go all the way down. Fill up and see where it goes and thanks for the info

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