From: "George (Andy) Anderson" 
Date: Fri, 1 May 1998 00:29:48 -0500
Subject: Re:  Wanted: 19 Tooth Speedo Pinion Gear and Sun Visor

On 30 Apr 98, Michael Foerster wrote:
> Does anyone have a 19 tooth (White) speedometer pinion gear?   I
> have a 17 tooth and my speedo is off.

Mike, having recently been through this myself, I have a bit of 
advice: what you really want to do is make sure that your odometer is 
correctly driven by the pinion gear.  Probably the easiest way to do 
this is to find a road with mile markers and see how much your 
odometer varies from the mile markers over, say, 6 to 10 miles.

If your odometer is really 11.7% *high*, then going from a 17 tooth 
pinion to a 19 tooth pinion would correct it.  Note that increasing 
the number of teeth on the pinion gear will decrease the indicated 
speed and mileage.

Once you have the odometer calibrated correctly, you can check the 
speedo to see if it is off.  If so, you can remove the speedo from 
the car and calibrate it with the small spring that returns the 
needle to the rest position (0 mph).  Once you remove the speedo from 
the dash pod, you will notice that the pointer return spring is 
attached at one end to the speedo pointer, and at the other end to a 
small arm the is held in place by a small nut concentric with and 
underneath the speedo pointer.  You might be able to move the little 
arm without loosening the nut, or you might have to loosen the nut 
*slightly*.  If yours is like mine (I have an '83 280zx n/a, but 
with a '79 280zx, 130 mph speedo), a small movement in the little arm 
will result in a moderate change in the tension on the spring which 
in turn results in a moderate change in the indicated speed for a 
certain input speed from the pinion gear.  This does, unfortunately, 
involve a bit of trial and error.  What I did was calculate how far 
off my speedo was, percentage wise, which turned out to be about 13% 
too high, then used a plug-in type electric drill (not a battery 
powered one; you need to be relatively reproducible) to spin the 
speedo (you need a reversible drill turning in the reverse 
direction), noted the indicated speed, and adjusted it until the 
indicated speed was 13% less than that. YMMV (no pun intended).

This takes a lot of words to explain, but if you look at a speedo 
unit for a while and play with it a little, it should become clear.  
You might consider getting a speedo unit of the same year as yours 
from a junk yard just to play around with before doing anything to 
the one in your car.

Good luck!