You might not always know what is going on under the hood of your car, but thanks to the wonders of electronics, you should be able to find out fairly easily, if you have the Griffin CarTrip OBD-II Hardware Interface. This device hooks up to your car’s OBD-II port and communicates with your iOS 4.0 (or higher) device, giving you information related to car mileage or even a real-time efficiency readout, thanks to the CleanDrive app. It’ll set you back $89.99, but that might be cheaper than calling a mechanic to save you when you’re stranded by the side of the road due to your car breaking down.