I had to wait about 3 months for my four 365/70 tires to be made and shipped. The drive axles 315/80 were no preblem. When they all came they were all less than 5 weeks old.
Manufacturers don't make all tire sizes all the time, they make a guess as to how many will be needed for new rigs and replacements for the year, then produce that many. Sometimes they guess low and a shortage exists till the next production run. Michelin holds some back for emergencies like a blowout but won't release them for just replacements.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '14 Jeep JKU Dragon Edition