Fill the gas tank of the automobile completely and then record the odometer reading. The odometer is a meter that keeps a running total of miles the car has driven and is located near the speedometer. For example, the reading might be 25,300 miles.
Record the odometer reading once the gas tank is nearly empty. For example, the reading may be 25,540 miles.
Subtract the initial odometer reading from the final odometer reading to get the total miles driven on the tank of fuel. Performing this step, you get 25,540 miles minus 25,300 miles, or 240 miles.
Divide the total miles driven by the fuel capacity of the automobile to get the gas mileage. Assume a 12-gallon tank for the sample exercise. Continuing the example, you have 240 miles divided by 12 gallons. This equals 20 miles per gallon.
Divide the total distance in miles for a trip you wish to make by the gas mileage to arrive at the total gas needed for the trip in gallons. If you are taking a 500-mile trip, for instance, you have 500 miles divided by 20 mpg, or 25 gallons of fuel.
Divide the number of gallons of fuel by the car gas tank size to get the number of tanks of fuel needed for the trip. Completing the exercise, you have 25 gallons divided by 12 gallons, equaling 2.1 tanks of fuel.
