The other day I picked up a rental car while visiting Los Angeles. In just a few days it ended up costing me well over the price of gas it would have taken to drive my own car there and back. A smart tool called CostToDrive would have helped me figure this out before I made the trip. It calculates how much a trip is going to cost you based on how far you're traveling, combined with the fuel efficiency of your car and average price of gas. Assuming you have to fill up when your tank is about empty, it tells you preciselytoo.
The tool has records for several types of vehicles, going back to 1999. If you've got an older vehicle or one that's not on the list, you can manually plug in both how big your tank is and the general highway mileage. From there it can do the math and give you the magic number.
There are a few things to note with this system. The first is that this is currently for the U.S. only, and your mileage (literally) may vary. The tool does not take into account any driving you'd do once at your destination, and city and highway mileage can be drastically different depending on the vehicle. Also, estimations are currently only one-way, which means you'll need to double the price if you plan on coming back.