Digital Speedometer for Police Vehicle Applications

Previously, a mechanically driven analog display speedometer was used for police vehicle applications. The speedometer required special processing to meet the tight accuracy requirements of ± 4.83 kph ( ± 3 mph) over a speed range of 32.2 kph (20 mph) to 160.9 kph (100 mph).

For the 1991 police vehicle, the speedometer accuracy requirements changed to ± 3.22 kph ( ± 2 mph) over the same range. This change, along with the speedometer design criteria, resulted in the change from a mechanically driven speedometer to a digital speedometer. This digital speedometer was packaged in an existing cluster design in a timely manner and for a minimal investment. Special design considerations included minimizing radiated emissions in the police band frequencies and allowing only the speedometer to be turned off during special nighttime driving conditions.