Speedometer Theory

[tweiss3]

I am a bit confused about how our speedometer reads. After some playing around in the snow, I have observed a few things:

1) Speed apparently isn't read by transmission speed or the rear wheels, cause I can spin them or lock them and the speedometer won't read.
2) It doesn't read from the front wheels either, cause you can lock them while moving, slide, and the speedo still reads.

I had always thought speed was determined from a cable from the trans, but maybe not?

91 S5 NA. I don't have ABS, which is usually the other way cars read speed.

Can anyone enlighten me on the operation, I'm just curious and its bugging me.

On another note, my S5 handles like a charm in the snow, predictable, responsive, and hooks damn well, even when the temp is 10*F.