Fill your vehicle's gas tank completely with gasoline and reset the trip odometer to zero. If your vehicle does not have a trip odometer, write down your vehicle's current mileage.
Drive your car until you need to refuel.
Refuel your vehicle's gas tank completely with gasoline and write down the amount of gallons you pumped and the price per gallon.
Write down the amount of miles driven between fueling. If your car has a trip odometer, write down the amount of miles displayed. If it does not, subtract the current mileage from the mileage documented when you initially fueled your vehicle.
Divide the amount of miles driven by the amount of gallons you pumped. For example, if you drove 315 miles and used 16.21 gallons of gas, you divide 315 by 16.21. Your result is 19.43 MPG. If the current gas price is $2.85 per gallon, you can drive 19.43 miles for $2.85.
Divide the number of miles to your destination by your MPG. If your destination is 450 miles, divide 450 by 19.43. Your result is 23.16. This is how many gallons of gas it will take for you to reach your destination.
Multiply the amount of miles by the price per gallon. For instance, the price per gallon is $2.85 and you need 23.16 gallons of gas, multiply $2.85 by 23.16. Your result is 66.0. It will cost $66.00 in gasoline to drive 450 miles.
