61 pontiac. the plastic gear in side of trans is accurate for the tires and rear gears. the odometer is accurate. however, the speedometer is slow by about 15%. the sweep is smooth, just reads slow. i think the mechanism is a traditional setup with a spinning magnet inside a metal disc thing, and the needle has a light spring to bring it back to zero. could the spring be not calibrated and holding it back too much?
Yes, or the magnetism is off a bit.... You can try moving the spring anchor, a little goes a long ways. http://www.jalopyjournal.com/forum/showthread.php?t=630833
X2 on what squirrel said. You are right if one function in the head is accurate and the other function isn't that s got to be where the issue is. There are a lot of good shops out there specialized in gauge repair / calibration.
If your speedometer has lost magnetism, it has likely not been serviced for some time. I strongly recommend doing so as the internal wear accelerates exponentially the longer it is in use. A professional servicing should include: full dis***embly with proper tooling, tolerance measurements and replacement of non-spec components, boosting of magnet, re-***embly with proper lubricants and settings and a final calibration/bench test. The model of speedometer movement you have is notorious for magnet shaft and lower jewel wear. Both of which can break other components (cable and speedcup spindle, respectively).
How about you do all that work to a speedometer, and show us how it's done and what's involved with pictures and captions? If we can see how much work it is, and the effort you put into it, we might become customers....
I'm still considering that scenario and have noted your previous suggestion to do so. It is simply a busy time of year and we have higher priorities right now. Shopping for a bigger shop as we speak (no pun intended). Plus, if I do start receiving more work, I'd have to hire and train another guy! That is a whole other mess.
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