The real world fuel consumption register is an excellent idea. However are drivers posting the figures from their computer or by keeping a check of fuel used and miles covered and calculating from those figures?

I have an Audi A5 2.5tdi Quattro. The computer shows fuel consumption of around 43mpg but I also log all the fuel used on the Fuel Calc app and this calculates the real consumption as around 35mpg.

This is an unacceptable difference and if typical of many cars we are all being massively mislead as to the true fuel consumption of our vehicles.