I converted from the 4L60E to the ZF 6spd. The ZF has just a 2 pulse sensor, the automatic tranny a 40 pulse.
I just googled, I thought speedometer is just another word for tachometer, my mistake.
(in Germany, we also have the word "tachometer", which is the thing that shows the speed, not the revs)

I'm also not sure, why the speedo works, but all other vss related things like cruise control, radio or logging don't...