u r confused in taking mileage,
choose 1 pump having same pressure, fill the tank , drive ur car till red light , note the mileage, again fill it from same pump, drive the car till red light and fill it from same pump, then u will be able to calculate the mileage.
yesterday i was going pindi, as cng is not in punjab so i filled my car cylinder first then after driving 12 km i filled it again from last possible pump and it was filled in 200 rupees, doesnt it mean i got 12 km in 200 rupees gas,