OK, here's the update. I was able to adjust the indicated speed on the gauge but not enough to make it accurate. When I purchased the unit I thought it was an original gauge, cleaned and lubricated. Instead, It's more like a hybrid, an original face and housing with what I think is a reproduction mechanism. This mechanism is adjustable which may be different from the original which I have been told in PMs is not. I'm awaiting contact from the seller to verify whether the mechanism is repro or original.
My guess is that the repro units need to be adjustable instead of using different gears, as on the factory gauges, so that they work with a variety of combinations of transmissions, rear ends and tires. Obviously they don't have enough of a range for all applications. Looking back at the information I read in the Master Parts Catalogue I need to find an original gauge from a 1 Ton or a 1-1/2 Ton truck. Guess I'll be on the hunt once again!
If anyone wants a picture of the adjustable mechanism please PM me with your email and I'll send a picture.