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I have 2 of those 4000 rpm tachs. One is NOS still in the DELCO box and the other is in a gauge cluster. Your will need a signal conversion box to make it work. I have that too.

If you are not sure about which drive gear you have, simply remove the driven gear and housing and look in the hole with a flashlight. The color of the drive gear will indicate which one you have. Might have to squeeze your head in close, but it works. Or if you have one of those snake...

Here is another site which shows the GM part numbers of the gears and housings. http://www.73-87.com/7387garage/drivetrain/speedo.htm

Here is the most helpful site I have found. You can calculate your exact driven gear (if you know your drive gear) based on axle ratio and tire diameter. Or scroll down to see the different gears and their corresponding color and tooth count. Note - in our 4WD trucks the gears are in the tail...

The OEM drive gear in the NP208 is a grey 15-tooth, GM #14037997. The OEM driven gear in M1008 and M1028 is a yellow 41-tooth. No doubt thus combination was for the stock tires and wheels. Not sure about M1009. Regarding the Drive Gear, fewer teeth will slow down the speedometer reading and more...