I&#8217;ll first say that I love the innovation and idea behind this product.  The idea is that every modern vehicle has an On-Board Diagnostics II (OBD II) port.  The port is usually located near the driver&#8217;s left knee, sometimes under a cover or door.  The Automatic device plugs into this, which may, as in our case, require that you completely remove the access door (since once the Automitic Link device is plugged in, the door won&#8217;t close anymore).  Car servicers and emissions testers connect their diagnostics computer to this port to clear codes and to look at numerous aspects of the engine&#8217;s performance. For most cars, this includes RPM, transmission temperature, coolant temperature, fuel flow, oil pressure, and many other things that show up on your dash display like fuel level.  All of these parameters can be queried by a third-party device, such as Automatic&#8217;s device or a ScanGauge device (which is more customizable and can pull all of this information and can calculate mileage too).  Both the Automatic Link and ScanGuage products are powered by the OBD port.While ScanGauge pulls a lot of information from the on-board computer and allows you to customize the windows and displayed information, the Automatic Link displays only gas mileage.  It&#8217;s not clear what other information the device may pull to do its mileage calculation, but it is clear that you cannot customize the interface on your iPhone to see anything else.  This makes sense when you consider that the Automatic device and its required iPhone (or Android) companion app are all built around helping you drive more efficiently.  However, this product is likely to appeal to tech-savvy folks, who will expect access to more information.  So here is a run-down of functionality, pros, and cons:- Setup was easy.  It automatically detected the vehicle type and VIN.  The con here, of course, its that if you are a privacy-aware, you might expect that this info is being sent to a server somewhere.  You MUST create on online login to use the app.- The iPhone app links up via Bluetooth.  You can leave the Link device plugged in all the time, so that when you get to your car, it links right back up.- Battery drain on the iPhone was acceptable.  There is no real-time information displayed while driving, so you can turn your iPhone display off while driving to save battery.- Each trip, no matter how short or long, is broken into segments, each of which is easy to navigate to on the home screen.  Information saved and displayed for each segment includes miles driven, mpg, fuel cost, time travelled, travel map, and your driving score.  The problem I had was that the mileage calculated was never anywhere close to reality.  I verified this on a long trip where I used the device from fuel stop to fuel stop.  I refilled and calculated refill gallons against odometer miles.  I got 14 mpg by my calcs, and Automatic calculated 7 or 8 mpg each time.  This is a huge difference and presents a major problem for a device that is all about calculating your mileage and helping you with fuel efficiency.  I checked my vehicle&#8217;s compatibility online, and Automatic confirmed that it would detect fill-ups and be able to calculate my mileage within 10%.  Based on what other reviewers are saying, it sounds like this device can be accurate...  Perhaps accuracy varies by vehicle manufacturer and model.- The unit calculates your driving score (0 to 100), and I was always getting 100.  I&#8217;m sure I&#8217;m not that perfect, some fine tuning on this algorithm would be good.  Note that the app does NOT show your driving score or mpg as you drive.- The Link device makes an audible sound when you go over 70 mpg, brake hard, or accelerate too fast.  This alert behavior was a little erratic when I tested it out.SUMMARYGood idea, but with mileage being as far off as it is, some refinement is needed.  I&#8217;m pretty sure I won't save $100 gas by just using this device, so I think the developers need to broaden the appeal by allowing users to access more information.  Perhaps they could allow users to set up other screens (access with a swipe) to see other information such as tranny temp, coolant temp, rpms, and other tings that the on-board computer provides.  This is a fun gadget with a lot of potential; just needs some refinement and better mpg calculation.