GM doesn’t and has never produced the duramax with anything but 3.73’s. The duramax is an option. 4.1:1 gears will be listed as standard equipment and along with the duramax under optional will be 3.73’s.
Wish one of the gm dealers would respond lol none of them have 4.10’s
You're mostly correct but because of so many option levels, mid year changes, and customer custom ordered espcially on work trucks we don't go by the thinking that this is the only way they come. It's best to know what they have by looking up the vehicle build on our special GM website. When we order parts for vehicles we don't tell the parts guy what gear ratio we need or what color a part is. We tell them what we need like a "ring and pinion" or a "door panel" and give them the last 8 of the VIN. They then look up the parts by that vehicles build sheet and give us the part it was built with.
If you want to know what your vehicle was really built with call your local GM dealer parts department and ask them. They will need the last 8 numbers of your VIN to look it up.