Determine your car's miles per gallon and the number of miles you are going to drive. For example, assume your car gets 30 miles per gallon and you are going to drive 400 miles.
Divide the number of miles you will drive by the miles per gallon. This is the needed gallons for the trip. In the example, 400 miles divided by 30 miles per gallon equals 13.33 gallons.
Multiply your gallons by an estimate of how much gas costs. In the example, assume gas costs $3.50 per gallon, so $3.50 per gallon times 13.33 gallons equals a trip cost of $46.66.
