Another No-Mod Tire Fitment Question

I ran through this with 72bluNblu a few months back. The difference added buy a 8 3/4 when swapping out a 7 1/4...
I measured +12.7mm (.5") per side.
13mm is correct. If he's telling you a 7 1/4 is the same dims drum to drum as a 8 1/4. (I don't have one).