I have a 91 Explorer that I have a question about. I have verified that when the speedometer is reading 66 miles per hour, I am actually only moving at 60 mph. I have verified with comparing to another vehicle and also with my Garmin. Both of those readings correlate with each other. No tires have been changed for about 3 years. I hadn't noticed this difference until my wife who just happened to be following me, wanted to know why I was driving so slow. When driving on our highway to our house, there are mile posts every mile. I have compared the miles traveled digits over a 10 mile distance and it is right on the nose. My question is this. Is it possible for the Speedometer MPH to be off and the miles traveled to be right on? I always thought they were all driven off the same gear on the transmission

There is only a certain degree of accuracy on many of today's vehicles speedometers. Your's is actually pretty typical.
Yes, the speed display can be off a bit from "exact" because it is a mechanical interpretation while the odometer is a digital representaion of the actual mileage. Better to be slower than the limit than as much over.

I realize that a speedometer can be off a bit. It wasn't before. My question was... Can the speed readout be off and the digital mileage readout be registering correctly?

Yes, that is possible.
If your speedometer doesn't jive with the odo, that difference should remain consistent for the life of the vehicle. Should you find that your's varies, you may need to have the IPC software checked and updated if there is any, or perhaps have the cluster serviced by a certified repair facility.
The odometer is solid-state electronics and it is likely the more accurate. The speedometer is electro-mechanical and because of that, is more likely to have problems.