I'm about to replace the seal around the speedometer cable where it attaches to the transmission.  I've read conflicting and confusing post about the cog or pinion gear.  Does the speedometer cog go with the transmission or the differential?  Should I put the '72 4 speed cog in the wide ratio 5 speed I just installed to keep the speedometer accurate?  Or leave the 5 speed cog in the 5 speed since I didn't change the 180 differential?  Thanks for any help.

The number of teeth needs to match the diff gear set.  But the orientation of the housing (which side the hold-down slot is on), and maybe the diameter of the gear (I've seen reference to other sizes but haven't seen it myself), can change with the transmission.

I know the 5 speed is blue, I'll pull out the 4 speed and see if it's yellow or maybe black.  Thank you Zed Head, I "like this".  

From datsungarage.com, 

YELLOW is the 16 tooth for the 3.36 BLACK is the 17 tooth for the 3.54 BLUE is the 18 tooth for the 3.70 WHITE is the 19 tooth for the 3.90 RED is the 20 tooth for the 4.11 PURPLE is the 21 tooth for the 4.38

Well that was easy.  they're totally different in design.  4 speed is on the left and won't fit into a 5 speed I have taken the blue one out of.

People have cut new slots in the housing.  Easier to poke the little roll pin out and swap gears.

The list with colors and number of teeth is handy too for if your tire diameter is off from stock.  I managed to get mine switched out so the speedo is correct after being off for years.

It wasn't ever fun doing math in my head when I had a cop behind me.

Edited June 10, 20168 yr by ta240

19 minutes ago, Zed Head said:

People have cut new slots in the housing.  Easier to poke the little roll pin out and swap gears.

That's a good idea, thanks again!