How can i estimate gas?

I have used an online gas calculator based on distance from city to city and year and make of car and gas prices over country.. but my sister just traveled from TN to MI and she estimated $800 but it cost $1,200.00. I am paying for a trip and the gas mileage calculator estimated $630 round trip from TN to NY? Any ideas?How can i estimate gas?
Well, the possibilities for why it was wrong are:



1) Calculator was wrong about distance (unlikely)

2) Calculator was wrong about MPG of the car (gotten from EPA but that's not reflective of most people's driving styles and actual MPG)

3) Calculator was wrong (or outdated) as far as gas prices along the way



I'd guess it's #2. The best thing you can do is find out how many highway MPG you actually get in the car that you'll be taking, and use that in the calculation.How can i estimate gas?
You will get a better calculation of gas usage while you are on your trip. Make note of how much it cost to fill your tank. Reset your trip mileage to zero. When you stop to fuel up calculate amount of gas used versus the distance you traveled. Use that as your base for the remainder of your trip.

professor layton