The primary difference lies in data sourcing and compatibility. OBD2 HUDs connect directly to the vehicle's diagnostic port, providing real-time data such as engine RPM, water temperature, and fuel consumption, but are generally limited to vehicles manufactured after 2008. GPS HUDs use satellite signals to determine speed and altitude, making them universally compatible with all vehicles, including older models and electric vehicles (EVs), though they may lose signal in tunnels or underground garages.