Having recently fitted a Nav system to my car, it appears that there are 2 ways that the vehicle speed is reported to the head unit/nav system.

One method uses a sliding voltage which I suppose is generated by some type of dynamo affair attached to the prop shaft or speedo cable.

The other uses a pulse (usually either 4 or 8 pulses per revolution).

My car had neither (deep joy) so I was forced to fit a small transducer in the speedo cable. This gives 4 PPR and the Nav system will autocalibrate this to the GPS signal over time so that in the event of GPS signal loss, the speedo sense allows the system to do dead reckoning in conjunction with the map overlay, gyroscope and compass it has built in.

It all seems very accurate and tells me that my cars speedo reads 100mph when I am really only doing 95. This is apparantly normal as the UK regs say -0% to +5% for speedo accuracy.

It is reported that in some makes of car this signal is factory routed to those 2 connector blocks that go into the back of the ICE unit.

I would like to volunteer Richard Lovejoy to the work for Speed Dependant Volume Control .His Voladj Kernal is superb and seems to do all the hard stuff, we just need a way to count 12V pulses on one of the serial pins and process the volume accordingly.

The other benefits of this connection could be a Speedo reading visual, trip computer with average speed, distance (or fastest 0-60 time for the boy racers) etc. The list goes on.

MK2 Red S/n 949