From past forum discussions, I came to realize that we don't really have a speedometer "expert" on this forum, but I thought I would throw this out there anyway. My speedo was bouncing around wildly, but was restored and now is solid/steady as a rock. I have 25.9" tall tires with what I believe to be a 2.71/2.76 ring and pinion. The 904 has recently been rebuilt.
According to mile marker check on the highway, traveling 100 miles is reading 106 miles on my odometer (6 divided by 100 = 0.06) That discrepancy is easy to figure out, the odometer reads exactly 6% too high.
According to the neighborhood "courtesy" radar checks (i.e. before school zones), at an indicated 30 MPH on the speedo I'm actually only traveling about 25 MPH. That is a speedo error of a more than 16% (5MPH divided by 30MPH = 0.1666) On the freeway, at an indicated 70 MPH on the speedo I am still being passed by traffic, so at that reading I figure I'm really only traveling about 60 MPH. Even at an indicated speed of 80 MPH I still get frequently passed, so I must be doing around 70 MPH. If I can find a "courtesy" radar check on the highway, I can better pin down the exact mismatch between indicated and actual velocity.
To point of my story and the mystery I have in mind (assuming the only problem mechanically wrong is the wrong speedo/transmission gear color) is how/why the odometer is off by 6% and the speedomenter is off by 16%. I would have thought the error percentage would have been equal.