The GPS-based vehicular speed limiting system comprises an on board diagnostic (OBD) interface unit and a GPS unit. The OBD interface unit plugs into the OBD-2 (or equivalent) connector inside the passenger compartment of a vehicle and communicates with one or more vehicle computers. The GPS unit receives signals from GPS satellites to determine the current vehicle location, which it may then display in map form utilizing map data stored in memory within the GPS. If the OBD interface unit determines that the vehicle is traveling faster than the posted speed limit, as determined by information stored in the map data and communicated to the OBD interface unit over the wireless link, the OBD interface unit may command the vehicle computers to limit the speed of the vehicle. Unplugging the OBD interface unit may result in a notification being sent to a smartphone.