Have you seen Automatic Link? It's a device that connects directly to your car's onboard computer and relays the data to your iPhone or Android device. The app can then give you feedback about your driving—if you then act on that feedback, your mileage could improve. You also get detailed diagnostic information, the kind of information normally available only to your mechanic, including decoding your Check Engine light. Cool, right? The device is available now for US$99.95 (no subscription required), it works only in the U.S., and it works with, "just about any gasoline engine car sold in the U.S. since 1996." See our review for more information.
Check It Out: Automatic Link Talks to Your Car's Computer, Reports to iPhone