Boy that's crazy. If it did any in-town driving at all it would have to spend a lot of time over 100 MPH to average out that high. Even an average life speed of 50 MPH would be amazing (and would need only 84K miles).
Your miles are about right for a 12 year old truck, but hours are about 60% of what they should be. You've either lost hours or gained miles.
Several usual suspects that would NOT do this:
1. Oversized tires would make miles go down and the avg MPH go up.
2. Likewise running the speedometer back would make MPH go up not down.
2. Odometer hasn't rolled over 1 million and started over. that would be 677 mph for these hours (assuming the odometer goes up to 1M before resetting??)
A. perhaps the part of the instrument panel/computer that measures engine hours has been swapped out with one from an later model truck so that the hours don't belong to your truck (although I would think both the hours and the miles would reflect that of the pirated truck).
B. Perhaps an electrical problem PREVENTED power going to the hour meter for a good part of it's life? I think you are missing 800-1000 hours
What an intriguing problem. I'm putting in a call to Sculley and Mulduer.
2019 Silverado 2500 4WD, Cajun Red
Michelin LTX M/S, bed liner, Alum bed box
Boost mirror turn signal jumper
Trans cooler grill