In order to get the best automobile gas mileage for your car, first you will have to determine and estimate roughly the gas mileage that your car currently gives. Then use this as a baseline to work upon, and then accordingly improve the overall automobile gas mileage. Fill the gasoline tank, and then record the readings of the odometer, drive till the tank is drained, and then again read the odometer when it is fully empty. Then proceed to fill it up again and determine how many gallons are required to fill it up. Now deduct the reading of the first odometer from the second and divide it by the amount of gallons to get the MPG.