Calibrating speedometer

I received a couple of metric speedometers from a fellow Z nut in Australia, an early one (same font numbers as the 160 mph 240z speedo) and a later one. Both go to 240 km/h. He said the early one wasn't working properly but that the later one did. I wanted the early font so I was going to put the early face on the later speedo mechanism. I then noticed that the sweep of both speedometers was different.

I decided to dismantle the early speedo to see why it wasn't working properly. I found a hair in it and some dust. After cleaning I reassembled and tested it with a Q-tip and my drill. The needle now moved smoothly with no jumping. To calibrate it I ran the newer speedo on my drill and noted the speed then ran the older one and adjusted it to match. I also ran a mph speedo I had just to be sure.

I then installed it in the Z and using a GPS noticed at 100 km/h the speedo and GPS match but at 35 km/h the speedo displays 20 km/h and at 130 km/h the speedo displays 135 km/h or so. On the speedo there is a rating showing 60km/h = 637 RPM. Maybe I should have calibrated it so 60 km/h matched newer speedo? That would give me accuracy at lower speeds.

Does anyone have any experience with how accurate these speedometers are throughout their range? I'd like to experiment with the return/tension spring on the needle shaft to see what affect it has on the speedo reading over it's range.

I really don't have as much time on my hands as it appears I do.

Edited June 17, 200915 yr by Jeffrey