The digital speedometer accepts an input from some sort of signal source- more on this subject in a bit. This signal source is generally a speed sender (sometimes called a pulse generator or electonic speedometer pickup) located in the transmission, the vehicle's PCM (power train control module , ECM, computer) or a GPS sender (which can be mounted in the vehicle or the gauge itself). Old style mechanical speedometers used a cable directly connected to the tailshaft on the transmission, transfer case or even a wheel hub. While these are still in use today in the aftermarket, the trend has been going electonic for quite some time. The benefit of electonic speedometers is they can be custom tailored to the individual vehicle by setting DIP-switches, programming via the LCD or hard programmed at the factory.
The reason for the increase in use for the aftermarket community is due to several factors:
1. Many if not most of the components for building today's enthusiast built vehicles are newer, many donor vehicles are form the 80s and 90s when the OEs started using electronic speed senders to monitor vehicle speed for cruise control, emissions, etc.
2. With the wide range of vehicle types being built today (pro-touring, tuner, race, off-road/Jeep) the need for a speedometer with a wider range of operation was required. Also this wide variety of vehicles being built by today's builders leads to a wider range of maximum speeds. A vehicle that spends some time on the track may want a maximum speedo of 140, 160 or even 200 MPH, while a lifted truck or rock crawler will want more resolution whith a maximum of 80, 100 120 or 140MPH.
3. The international market has really opened up and calibration of the speedometer is critical in proper speeed reading of metric (km/h, KPH) speeds. Here at NVU we can load a metric speedometer program creating a suitable product for use overseas without needing to change magnetic wheels or odometer gears on a mechanical unit.