View source for The 'Tracking Inquiry' - Yorkshire Ripper

On the 6th February 1977 an exercise, known subsequently as the “Tracking Inquiry” was mounted to identify the vehicle which had left the tyre marks at the [http://crimehub.co.uk/index.php?title=Irene_Richardson_-_Yorkshire_Ripper_Victim Irene Richardson] murder scene. In the preliminary stage it was thought that any one of 100 different types of vehicle could have been involved. This number was determined on the basis of the "track width" indicated by the distance between the tyre marks at the scene. 

As a preliminary step towards a formal inquiry intended  to identify the vehicle, night duty officers in West Yorkshire were asked to examine vehicles during the course of routine patrol. Each time an officer examined a car he recorded the registered number and tyre details and a card was subsequently completed showing that that car had been eliminated from the inquiry. As a result of the application of a more refined selection system the list of 100 possible vehicles was reduced to 51. Because "back record conversion" at the Driver and Vehicle Licensing Centre and the Police National Computer was at that time incomplete, inquiries to trace registered owners of relevant vehicles had to be undertaken in local Vehicle Licensing Offices as well as with the Police National Computer. 

This exercise produced a list of 53,000 registered owners of the 5l different vehicle models living in the West Yorkshire Metropolitan area and the Harrogate area of North Yorkshire. Sutcliffe and his white Ford Corsair were amongst the 53,000. When the vehicle owners list became available, the cards completed in connection with the "night-duty check" were checked against it to eliminate all vehicles which had already been examined by the police. Index cards were then completed for the remaining vehicles to be checked by direct house visits but the exercise was aborted after the attack on Maureen Long in July 1977, when 20,00D vehicle owners remained to be seen. 

Sutcliffe was one of the 20,000 owners who were not seen.