This page will calculate how much you will spend on gas on a road trip.
This only calculates the price for gasoline. The price of parking, wear and tear, road tolls, motels, tickets, etc. is not included.
Efficiency / MPG
Keep in mind that people don't usually get the advertised fuel efficiency for their car. To calculate your actual miles per gallon, reset your trip meter when you have a full tank. The next time you fill up your car, divide the number of miles you've driven with the number of gallons you just put in your tank.