to work out ur MPG divide however much it cost u to fill up by what it cost u per litre i.e.
£43 (cost) % .789 (cost per litre) = 55 (how many litres u filled up by)
Now divide 55 litres by 4.5 to get the total in gallons
55l % 4.5 = 12 Gallons
now divide the totals miles since ur last fill up by the number of gallons filled up by this time. In my case this would be:
512 miles % 12 gallons = 43mpg