Thanks to Joshua Teixeira for the following :
Thanks to Gary for assisting with my speedometer calibration. Gary sent the following to me privately:
>Here's what I know so far: > >The movement, as you describe it, is similar to a D'Arsonoval meter movement >with the clock spring. Clock springs are used because that have a very constant >force thru a wide range of wind, so the meter movement can be very linear. They >are zeroed by just moving the spring-end as you found, but the full scale is >adjusted by changing the length of the spring: Longer spring means a lower full >scale (speedo reads higher then actual speed), shorter spring means higher full >scale (speedo reads lower then actual speed). I have re-calibrated meter >movements by opening the clamp on the clock spring and moving a bit more or >less of the spring into the movement. > >Your speedo reading too low would mean that the spring is too strong and needs >to be lengthened. Hopefully there is enough spring outside the clamp to >lengthen it. > >Let me know what you find. If I can remember, I'll check the shop manual >tonight.
This confirmed what I had learned earlier about adjusting the 'zero point' of the speedo from taking apart an 'extra' Tempo speedo I had laying around. I did not know that the length of the spring itself could be adjusted, and that this would effect the 'spring constant'.
To recap, the anchor for the return spring for the needle is adjustable, which adjusts the motion of the needle relative to the numbers on the dial. Thus if your speedo was out by the same amount across the entire speed range, then adjusting the anchor point only would solve your problem.
Fortunately for me, this was all I needed to adjust my speedo which was reading 7 km/h below my actual speed. Adjusting the length of the spring involves some pretty tricky work with forceps, due to the delicate nature of the spring and the wedge holding it in place in the anchor.
The Steps were as follows: (92-95 I think, others similar)
Socket set with Torx bits for screws in 'shield' below the steering column.
Battery drill with socket adaptor, extensions, and a universal joint.
(Drill optional, can use socket handle or Torx screwdriver)
Torx bits for the battery drill. (Optional, can use Torx screwdriver)
Tweezers or dental pick
10 lb or lighter monofilament (fishing line) - can use string that will hold a knot without pulling out.
Stop watch with helper to operate it
1. Wash your hands, as you will be handling objects that you will be staring at for the next 250K miles - you don't want any fingerprints or scratches on stuff you can't clean without disassembling the dash.
2. Remove the 'shield' below the steering column - 4 screws, torx head. This uncovers two screws for the dash.
3. Remove the column shroud, bottom piece first. Remove the lock cylinder by putting the key in the RUN position, depressing the small pin recessed into the bottom of the cylinder, facing the floor, and pull the lock cylinder out. Remove the top shroud, and re-install the lock cylinder to turn the car back off, or to accessory to listen to tunes.
4. Remove the dash portion around the cluster. There are a number of push pins in addition to the four screws (two over the cluster, and two at the bottom, which were uncovered by removing the 'shield')
5. Disconnect all the switches in the dash portion so you can put the dash portion in the back seat. A small screwdriver is helpful for the defroster and light switch. You can remove the cluster as a unit, and work on it on your lap, or disassemble it in place. I chose to disassemble it in place, as you have to put it back in anyway when you are road-testing and adjusting. The following instructions are for loosening the cluster.
6. Loosen the cluster by removing the four screws holding it in place. The cluster should now slide towards you a small amount.
7. Work your hand behind the cluster to disconnect the speedo cable from the speedo head. The is a very tight squeeze, and the sharp corners on the circuit boards will dig into your hands. The cable is released by squeezing on the tang and pulling back slightly away from you with the cable.
8. Remove the clear plastic cover from the cluster. Set this carefully out of the way, or give it to your wife to clean. Tell her not to scratch it, to only use soap and water or soft cloths and Windex. Alternatively, you can clean it yourself. Remember, you will be staring at any scratches you make for the next 250K miles.
9. Remove the two remaining screws that hold on the black plastic cluster mask. Note that there are two black rattle clips in between the mask and the clear cover. Don't lose these.
10. You should now see the speedo. Remove the two remaining screws and work the speedo out of the cluster. You will have to be careful with the part that sticks up behind it that has the low cooling light. Rotate this toward you and out of the way.
11. Observe the speedo, and note the calibrator's initials. Curse this person, if this makes you feel any better. Resist the urge to alter the odometer, as it sits vulnerable in front of you. Between the cup that is connected to the needle and the face with the numbers is another frame. Between this frame and the cup you will see a fine spring that is connected to the needle shaft. The other end is connected to an anchor that has two tangs sticking up on it. One of the tangs is connected to the end of the spring, with the spring wedged into place with a little brass cylinder.
12. Attach some monofilament (10 lb or lighter), using slip knot loops, to each of the tangs (two tangs, two strings, two loops), being careful not to kink the spring. Don't tighten the slip knots too tight. They should be just tight enough so as not to fall off. You will need to remove the loops later without disturbing the anchor.
13. Hold the speedo as you normally observe it, with the two strings hanging down. Pull the strings lightly toward you, and position them into the V at the bottom of the speedo.
14. Re-install the speedo back into the cluster so It just contacts the speedo cable, and not too far in to that the cable clips back on. Do not put the coolant light part back in, leave it rotated forward. Secure the speedo loosely with one screw. The two strings should hang down around the steering column.
15. Put all the pieces out of the way so you can go driving. You will need a helper with an accurate stop watch
16. With the car moving, make sure you can see the speedo internals moving through the window in the speedo, now vacated by the low coolant light part. If they are not, push the speedo a little farther in, until the speedo is on the cable.
17. Go to a sparsely populated road with mile markers where you can drive a constant speed for 10 miles or so at a time.
18. Set your cruise so that you are doing exactly 60 mph. This is done by adjusting your speed relative to the mile markers so that 1 mile takes exactly 1 minute, with an average taken over 5-10 miles or so.
19. Once you are travelling exactly 60 mph, then with your helper holding the wheel, take the two strings that are hanging down from the anchors and adjust the anchor. BE GENTLE. A little goes a long way. You pull on the right hand string to increase the indicated speed and pull on the left hand string to decrease the indicated speed. Keep gentle tension on the opposite string to steady your hand. There was at least 20 km/h of adjustment on both sides with my speedo.
20. When you are satisfied that the needle is reading exactly 60 mph when the car is travelling exactly 60 mph, then try 30 mph (1 mile = 2 minutes), 90 mph (1 mile = 40 seconds), 120 mph (1 mile = 30 seconds). Do not adjust the speedo at these other speeds, just note the actual readings to determine if you need to adjust the length of the spring. I was exactly right on at 60 km/h (37.5 mph) when I adjusted the speedo at 120 km/h (75 mph), and my co-pilot (future brother-in-law) holding the stopwatch was too chicken to go 180 km/h (112.5 mph).
21. If you feel the need for adjustment of the length of the spring as per Garu's notes, then good luck.
22. Carefully remove the two pieces of string. Tweezers or a dental pick is helpful. Be very careful not to disturb the anchor, or you will be going back to Step 12.
23. Road test again to make sure you did not disturb the anchor.
24. Go back home and retrieve the washed clear cover from your wife.
25. Re-assemble the cluster, remembering what screws go where - two for the speedo, two for the mask, the rest for the clear cover. Remember the fingerprints.
26. Re-attach the speedo cable from behind and screw the cluster into place. Remember to remove any finger prints.
27. Re-install the dash, remembering to connect all the switches.
28. Remove the lock cylinder and install the top of the steering column shroud. Re-install the lock cylinder.
29. Install the bottom of the shroud, noting all the clips etc.
30. Install the 'shield' below the steering column.
31. Enjoy an exact speedo. If you get stopped for speeding, tell the officer that you took the trouble to have your speedometer calibrated (note I did not say 'say that YOU calibrated it), and when was the last time he calibrated his radar?