So I'm an idiot and when I replaced my odometer gear I forgot to take note of where the Speedo needle sat. Now it's 20 kilometers off. Is there an index mark on the Speedo face to zero the needle? I see a couple white marks but don't know which is which.

 

Not really.

There's also no "index" for where the meter movement goes when it's not powered. Meaning, even if there was a mark on the scale (that's its name, not "face"), it wouldn't be useful for anything anyway.

Best thing to do is pop the pointer (there's no "needle", only a pointer) back off, then with the cluster installed, turn on the key, and put the pointer back on pointing to zero.

 

Well, yes, there is in fact sort of a zero mark.
it's SUPPOSED to be that tiny white mark in the middle down there, but I took note of the pointer position before removing it and it was never at that mark.
Put it back on just like this and checked my speed against the GPS afterwards and it is so perfectly spot on...
That would be my advice now: Go out on the road, go 30 or something according to GPS, then see what the speedo says, stop the vehicle and reposition the pointer as necessary.

 

I actually ended up doing exactly that. It's now within 1 kilometer of where the GPS is.