OBD stands for on-board diagnostics. The OBD-II port enables external devices to access data on mileage, emissions, speed and other subsystems that are already stored in a vehicle’s computer. The port is most often on the driver’s side of the vehicle either on or under the dash. The OBD-II port is typically available on any vehicle made after 1996. While it’s most often used by mechanics to diagnose problems with a vehicle, there are a growing number of devices that drivers can install to monitor their vehicle’s performance. One potential capability of these new devices is the calculation of a mileage-based user fee.