The differance is US gallons vs. British or Imperial gallon measurment. 50.450 litres per 510.0 miles is 5.1 litres per 100 km, but the correction should be an increase from 46.0 MPG (US) to 55.0 MPG (British).
Actually, I do have UK units set, and more to the point I am entering the values in litres, not gallons. 510 miles is c. 820 km. 50.45 litres divided by 8.2 gives 6.15 litres/100km.
Since I am entering the volume of fuel in litres, it should eliminate the whole Imperial/UK gallon issue. The mpg is correct for the UK values. My overall l/100 km values are correct. But each time I fuel, the reading for that fill-up given to check your figures is wrong by 1l/100km.