Its very important to understand how much fuel was there before you filled in, the best way to check your car FE is tankful only only upto teh auto cutoff pls dont over load the fuel tank, also be very careful with the Pump guys they would tell you sirji round figure latta houn etc, always ask them to put the fuel in auto shut mode.
Once you are done with ur tankful then drive for 300km (if you can come to the same petrol pump, and teh same nozzel) or else any other fuel station fill again to autoshut mode and then u get the exact method.
Vehicle : Swift Vxi with ABS
Fuel : Petrol
Calculation Method : Kms method Observation by re-setting the trip meter every time I refuel to a certain level on the fuel meter.
City Mileage : 9.5-10.5 kmpl with 70% A/C On.
Highway Mileage : 14 - 14.5 kmpl with 70% A/C On.