The amount of fuel needed for a road trip is roughly calculated by dividing the total amount of miles to be traveled by the vehicle's miles-per-gallon (MPG) rating. The amount of miles required is found by using services such as Google Maps or MapQuest, while a vehicle's MPG rating is listed in the owner's manual or can be directly calculated.
The first step in calculating a vehicle's MPG rating is to fill up the tank and reset the trip odometer. The car is driven until at least half the tank is used, then the tank is filled and the amount of fuel used along with the trip odometer's readings are noted. The amount of miles driven is divided by the amount of gallons of gas put in the tank, and the number found is the vehicle's MPG rating.
The MPG rating can fluctuate depending on driving habits and conditions. A vehicle's fuel efficiency sharply decreases above 60 mph; keeping the vehicle below this speed improves its MPG rating. Also, changing the vehicle's oil, keeping the vehicle properly tuned and ensuring the tires are properly inflated reduces the amount of power lost trying to move the car, thereby increasing fuel efficiency.