I have a little method i use for calculating fuel consuption. It goes like this
Fill the car right up untill the bowser goes 'click'
Reset your odometer
Drive around untill whenever you want to fill up again (the longer the better)
Fill the car right up again untill the bowser goes 'click'
Record how many litres (X) and how many km's (Y) you did
Then follow this
Ltrs/100km's
Ltrs =(X/Y)*100
And you have how many litres you have done per 100km's
The longer you travel the more accurate the results will be. Also try and use the same bowser everytime to keep the variables as constant as posible.
In my 6MGE supra, i average 14L/100km's
My cortina last got 11.007L/100km's