If your vehicle do not come equipped with a trip computer, you can find out the fuel consumption figures by doing a simple manual calculation.
There are many methods if you ask around but I think this method should be the most accurate and simple to do.
What you need is to get a reference point. You can choose a marking on your fuel gauge but the easier way is to use the reserve indicator.
Once your reserve indicator lits up, reset your trip meter to zero. There is no need to fill up immediately. You can fill up at your earliest convenience.
When you do fill up, you don't have to fill up to the brim like some methods suggest. It is practically not accurate because of inconsistencies in what is defined as 'really full'. Some nozzles are longer than others etc. Also, it is a hassle.
Top up to an amount you are comfortable with. Once you have topped up, take note of the amount you filled up, in liters, not dollars. It would be easier if you fill up to a round figure.
Then travel as per normal until the reserve indicator lits up again. Take note of the distance you have traveled then divide by the amount of fuel you topped up.
Distance traveled/Fuel topped up = FC (km/l)
Fuel topped up/(Distance traveled/100) = FC (l/100km)
So say for example you've top up 45 liters and traveled 600km, then your FC is 600km/45l = 13.33km/l or 45/(600/100) = 7.5l/100km.
For the former, the higher the better and for the latter, the lower the better
It's that simple.