Step 1: Fill up your tank.
Step 2: Reset the odometer.
Step 3: Use the car for some time, for 50 KMs or 100 KMs or till half tank.....doesn't matter.
Step 4: Now go to refill the tank.
Step 5: When the guy stops filling up the tank to full, check the LITERS on their screen. Suppose it took 10 liters of petrol to fill up the tank, and ur odometer shows 120 KMs, divide this odometer reading by the Liters and u will get the exact reading. (in this case, it will come out 12KM/L)
Step 6: Reset ur odometer and repeat the above 5 steps for 3 times to get the mean value of your mileage..
Edit:
and congrats for the car bro......