Miles per gallon (MPG) is the most commonly used measure of gasoline mileage. It shows us how far a gallon of gasoline can take your car. You can use the following technique to calculate your MPG yourself: 
MPG = miles traveled/gallons of gas used
Measurement Method 
Comparatively, there are more indicators as to how efficiently your car is performing on fuel. It’s not some incredibly high-tech information that can only be discerned by checking some obscure figure at the next gas station.
Gas Mileage (MPG) in a Nutshell: MPG actually refers to miles per gallon hence the capacity that a car’s petrol tank has to power the car in terms of distance. This one informs us of the distance that one can cover in a certain number of miles, in our case the car, in terms of how many gallons of fuel it shall utilize.
The higher the value of a car’s MPG, the better the gas mileage of our vehicle. Small and energy-efficient cars offer greater fuel economy and result in more miles per gallon, cheaper at the pump.
Calculating Your Vehicle’s MPG: It’s easier than you think.
Try this the next time you fill up your tank:
Reset your trip odometer to zero. This little counter on your dashboard keeps track of how many miles you have left since the reset. 
Fill your gas tank full. 
Drive as usual. Check your driving mileage before refueling. 
Fill your tank again. Decide how many gallons it will take to fill the tank this time.