If anyone cares, the GPS unit maintained its accounting for both parts of the day which saw me cross the border, meaning it "knew" how far I had traveled in total, even though I changed out the SD cards in mid-trip.
Also, once in MX, it was able to locate addresses and data points, and generate point-to-point directions, with the same imperfect and occasionally infuriating "accuracy".
As imperfect as it was, it was good to have, because I couldn't find a map in a scale that would have been *really* useful, so using both the map and the GPS usually made for positive results.
Note use of the word "usually". As always, YMMV. Be safe and enjoy!
The Chief (tm)
Will Work for Fuel
1 Continent / 3 Countries / 4 Provinces / 11 Estados / 39 States