You can't go wrong with Dash, which provides both the free app (available for iOS and Android) and the hardware you'll need. In fact, it offers a variety of sensors, ranging in price from $10 to $99. Each device can pick up and record different types of diagnostic information—such as distances, routes, fuel consumption, and engine health—so make sure the product description says it can sense the stats you want to access. When you choose a sensor and put it in the OBD-II port, it sends this data to the app, which tracks, interprets, and shares the details. For example, if you break down, Dash can give you a report on what's wrong and how much it will probably cost to fix; if you're in a crash, Dash can automatically notify close friends and family.