I'm wiring a Stewart Warner G.U.S. tach and I need to know whether the signal from the ECM is digital or analog???

waveform?

Is the stock tach on the car digital or analog? Numbers or needles?

The output is more then likely going to be digital. ie if you read it with a voltmeter or osciliscope, it would read an alternating 0 and 5v. Or as mentioned could be waveform. I'm not really sure.

The stock tach is analog, but it has a motor that moves the needle. There is a controller in the combination meter that tells the tach motor how to move.

The output is more then likely going to be digital.
The more I think about this the more I believe your right. I suppose if the tach was considered analog there would be some sort mechanical mechanism moving the tach. More than likley the instructions are asking about analog in case you have an old car.I have installed two different brands of market tachs in an STI and never was asked that question.

it is digital; ie, it has two states, on and off. the speed at which it changes state indicates rpms.

Yeah, I think it's digital too. I'll let you all know once I get the tach in.