Abstract: A signal processing apparatus for a taxi meter comprises a processor (108) and a plug (122). The plug (122) connects to a vehicle's (100) diagnostic system (116) and receives an incoming electrical signal from the vehicle's diagnostic system (116). The signal passes to the processor (108), the processor which convert the signal to distance data indicative of a distance travelled. The processor (108)interprets the distance data into a form suitable for reading by a taximeter and transmits the interpreted distance data to a taximeter.