I decided to give the calculator a test run, as I just took a drive to Auburn, Washington, for work, and I know what I spent on gas for that little adventure. The first thing I found was that if your car is older than 1999, you won’t find it in the drop-down menu.

Cost to Drive couldn’t find my little red truck in the system, so I entered it free-form style, including the MPG and size of the tank.  It told me I could expect to spend $25.75 on that trip, which was just about right. But I could have figured that out in my head using the information it had me type in.

The builders of the site call the process “galculating,” which is what I did while stuck in traffic on Interstate 5 yesterday. “If I sit here for x number of minutes burning y gallons of gas, how much time and money am I losing?” There’s nothing like a traffic jam to make word problems seem fun.

Cost to Drive may help you decide if a trip is worth it, or it may help you compare the cost of gas to the cost of a train ticket, but the math is pretty easy. I firmly believe you could figure this stuff out without a dedicated web site, but if your brain is having a difficult day, then Cost to Drive may help.

MyGallons.com promises that users can buy gas at today’s price and fill up in the future. Once the service is completely up and running (right now, you can reserve an account but not yet use it), users create an account, receive a MyGallons card, and check the web site for the current price of gas. You’ll be able to click a button to prepay for as many gallons of gas as you like.

The prices quoted on MyGallons are all regular unleaded. If you fill up with premium or diesel, the company will adjust your balance to account for that. If you’re out of town and the price of a gallon of gas is significantly lower — or higher — than your hometown price, MyGallons will adjust for that, too.

A program like MyGallons is designed to hopefully save you money — not gas. Having a prepaid fuel-only card will not improve your fuel economy. You may in fact get worse mileage out of your car if you drive like you know you paid less for that tank of gas.

Something to keep in mind: Tara Baukus Mello, lead analyst for the NADA Guides, told me in an interview that they don’t expect gas prices to rise any more this year, and they may even drop a little. Not a lot, but a little. With the price of gas in such flux, I’d check my crystal ball twice before prepaying for gas.