So i wanted to upgrade to the 140 mph speedometer and the V6 tach to better match the LS1 in my car. Right after my swap my odometer stopped working. So I picked up a 140mph speedometer and low and behold its odometer is also dead.
So I ordered http://m.odometergears.com/mobile/products/Pontiac/Firebird-Trans+Am/178 and rebuild the odometer. But I wanted to verify I had the needle on right.

So I ordered a cheap signal generator that outputs the same voltage as the input voltage at the selected frequency.


Here is the results.


It's as accurate as it was before, I will replace all the capacitors before I install it in the car to be prudent.

This works for the tach also.

Speedometer is 4000 pulses per mile so the formula is.
4000*speed/60/60=frequency in hz for that speed in mph.

For a v6 tach it's 3 pulses per revolution(ppr), and for a v8 tach it's 4 per revolution.

So for example at 2000rpm on a v6 tach the math is.
2000rpm*3ppr/60=frequency in hz.


Speedometer wire, red 12vdc, black ground, green signal

Red 12vdc, black ground, green signal