Please do film the odometer rollover, and the 1 mile afterwards. I have heard multiple places report the instrument cluster dies at 1M or 1,000,001 miles, and I am curious if that is true, and what it looks like when it happens.
If true, I really wonder if it was intentional (to prevent rolling back to 0? just being dicks?) or a bug (trying to write 7 digits to 6 positions in RAM causes an overflow or similar).
I've been figuring out the GMT800 IPC best I can. I've mapped out where most of the data is stored:
If yours does die at 1M miles, I would be quite interested in taking a look at it to try and understand what goes wrong. Might even be able to fix it for you, though probably by resetting the odometer to a lower value (back to 0 if you want).