[avatar307]

Speed is calculated by a speed sensor attached to the transmission. A series of pulses is then sent from the PCM to the cluster, which controls the speedometer and odometer. (I believe pulses are how it is done.)

Odds are you will not be able to tap into the stock speedometer sensor wires, as you won't find a gauge that is completely compatible. Even if you pulled the speedometer out of another GM, it probably wouldn't work. If it did, it probably wouldn't exceed the 100 MPH mark. The PCM knows that is as high as the stock speedometer goes, and likely does not read or send any signals greater than that.

Best best is to install an add on speedometer which uses an isolated, seperate, dedicated speed sensor elsewhere on the car. That's the safest, and would give you the most accurate readings. (As you really don't want to mess up your stock speedometer line and create a false speed readout... Speeding tickets suck, especially when it's your fault for messing with the speedometer. LOL)

As for a Tach. It might be possible to tap into an engine sensor, but I expect they use an equally propriatary method for obtaining readings. It likely is read directly from the Camshaft sensor (as that sensor has two parts, one the reads speed and on that reads exact position).

However, REMEMBER that the Camshaft Sensor is critical for the functioning of your engine as a whole. It determines many varibles applied to your engine systems. If you disconnect it, as soon as your car reaches temp, the engine will die because it NEEDS that sensor input to function. Thus, tapping into this sensor is also likely a bad idea.

I'm really wanting to find an engine readout that functions of data from the OBD-II connector. As when connected to an OBD-II PCM, you can monitor every varible from the engine in real-time. Fuel pressure, air/fuel mix, RPM, speed, MPG, coolant temp, transmission temp, shift point, EVERYTHING.

I'm seriously investigating combining a pile of ICs and buuld a simply 20 character LCD display that would allow you to display and scroll through the varible readouts as reported by the PCM. Any electrical engineers here that want to get involved? Any thoughts, comments, suggestions?