Fill up your tank several days before leaving on your trip, noting the mileage on your vehicle. Drive around for a day or two, going about your normal routine. Then, fill up the tank again. Note how many gallons you put in and what your current mileage is and use this to calculate your vehicle's gas mileage. For instance, if you put in 10 gallons and your odometer shows that you traveled 250 miles, your vehicle gets 25 miles per gallon (mpg).
Calculate the distance between your destination and starting point, doubling that amount to get your round trip distance. If your destination is 300 miles away, the total road trip distance is 600 miles. Figure distances by consulting an atlas or online road map.
Estimate the number of miles you will travel while at your destination. Most likely you are not going to just drive to your destination and come back. What kind of activities do you plan on doing? Where will you be staying? Do you plan on going for a drive to take in some scenery? Estimate these distances and add them together. Consult the Internet for help or call your hotel and ask them how far away they are from some of the sites you wish to visit.
Research the cost of gasoline. Gas prices often fluctuate from place to place. See how expensive gas is at your destination compared with where you live. Add these figures together and divide by 2 to get an average cost of gas per gallon.
Add the total number of round trip miles with the number of miles you will travel during your visit. Divide this total by the mpg of your vehicle to get the total number of gallons you will use. Then, multiply the number of gallons by the average price of gas. You now have the total cost of the gasoline for your trip.
