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Hi there is a problem with the need of filling - up so to get detailed statistics about fuell consuption.
I own a bi-fuell car (lpg/petrol). The problem is that the fill-up of the lpg tank is relative situation :) . Theoritically it must filled up to 80% of the nominal capacity. But sometimes you can fill no more of 70% and sometimes (depending the fuell pump pressure and the remaing pressure at the lpg tank) you may fill-up beyond 80%.

And this screws-up the statistics. Another problem is at many cases I have  to use petrol when i run out of lpg, for some distance. How can exclude that distance from the calculation of lpg consumption?