OK, here you go.
1. Fill your tank till the gun clicks off so that it's topped off.
2. Reset your trip mileage counter.
3. Drive until you need to fill up again. Take it right down to 1/4 or so as you'll get a more accurate calculation the longer you leave it.
4. Fill up again as in 1.
5. Note the number of litres it took to fill up.
6. Divide litres by 4.55 to give you the number of gallons used.
7. Divide the number of miles travelled between fill ups by the number of gallons used.
8. Tell everyone on here your mpg
KenL.