|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! :)