Fill your vehicle with gas. Record the mileage shown on the vehicle's odometer in a notebook.
Drive the vehicle until it is nearly empty. Fill up the tank again. Record in the notebook the mileage and the number of gallons of gas needed to refill the tank.
Subtract the original mileage figure from the new mileage figure. For example, subtract the original mileage of 10,100 miles from the new mileage of 10,475 miles to get 375 miles. You have driven the vehicle 375 miles since the first fill-up.
Divide the result in Step 3 by the number of gallons needed to refill the gas tank. Continuing the example, divide 375 miles by the 15 gallons needed to refill the tank to get 25. Therefore, your vehicle's gas mileage is 25 miles per gallon.
Determine the distance you will drive on your road trip. For example, if you live in Chicago and are making a trip to St. Louis to see your beloved Cubs play the Cardinals, you would be driving approximately 525 miles round trip.
Divide the total number of miles you expect to drive on the road trip by your vehicle's mpg. To continue the example, divide 525 miles by the 25 miles per gallon your vehicle gets to arrive at 21, the number of gallons of gas you will need to make the trip.
Multiply the result from Step 6 by the price of gas per gallon. To complete the example, multiply 21 by $3.50 to get $73.50, which is the cost of the fuel for your road trip.
