Thanks for the input guys -- lots of food for thought there. Just to make sure we're in synch on terminology: when I talk about "speedo drive pinion" I mean the gray plastic gear that makes up one molded piece together with the little output shaft that drives the speedo cable at the OD unit.

I think that the speedometer is actually functioning as it should relative to the impulse it is receiving, and that it doesn't need cleaning, lubing or spring adjustment because both the odometer and the speedometer show the same degree of pessimism. The difference between a 0.99 and 1.03 would account for the 5% difference that GT experiences, but can't explain my 20%. Where, BTW, is that marking found on the speedo? [Yes, George, my Venezuelan wife shocks me with commas in our check book where I would use periods. So it's not just the Europeans. But then, I think we're the only country left in the world using feet, inches, gallons, and pounds. I wonder who is right ;-). And I also, after decades of hunting for the right words in Spanish, have become an expert circumlocuter.]
I have no idea of the origin of the M41. I took it out of a car that had also been converted to OD from something else. I'm also not certain of my differentials ratio - no tag on it. Most likely it is 4.1, but it could be 3.91 or 4.3. I can do the crankshaft rotation test, but whatever the reult, it still wouldn't account for more than a 5% discrepancy; but who knows how all the discrepancies can add up?
Meanwhile, I've heard from one outfit in the UK, and they actually have a 17 teeth drive pinion. I may just go for that and see what it does fo me.
Bob S.