Now here’s a cool do-gooder app: MyMPG is designed to help you use less gasoline by changing the way you drive. I know it sounds crazy but this app actually works.

Basically, MyMPG uses the iPhone’s accelerometer to monitor the pace of your driving. If it thinks you should ease off the gas or give the brake pedal a rest, it will emit a sound to let you know.

I spent several days driving with MyMPG, and it’s pretty cool. The audible feedback is accurate and convincing—I found myself immediately responding to its direction and changing the way I handled my car. MyMPG also includes a fuel log, which helps track your fuel use over time.

Unfortunately, the "stats" function only summarizes use over two months, six months, and one year, so I can’t really say how well this aspect worked.

I do love the fact that MyMPG doesn’t require a network connection. It works just as well in urban areas as out in the sticks.

I do have some complaints. MyMPG sounds just like some radar detectors, so if you have a cop-finder on your dash, prepare for confusion. Also, and this really stinks, the iPhone's iPod won’t work when MyMPG’s sound is enabled. You can disable sound, but doing so forces you to rely on the app’s visual feedback, and that’s not safe when you’re driving. 

So unless you must use your iPod's playlist while driving, you'll find that spending $2.99 for MyMPG will save you money.