Fill up your tank full.
	Reset the travel odometer clock
	Travel until you've almost conumsed a full tank.
	Fill it up full again.
	Record the number of Litres/gallons entered.
	Record the kilometres/miles travelled (from the reset odometer).

Please note that the accuracy of your results depends greatly on you following these steps.  The accuracy is not 100% anyway, but is within a 10 to 15% error margin.  This is due to filling up at the pumps, as it's hard to fill right up exactly and put in the exact amount you just used.

Please note: In Australia we talk in metric, however do refer to mpg referencing to English gallons not US.  There is a large difference in the two.  My program displays both though! :)