The manual for my 2008 says to replace when tread thickness is 1/32". My state requires replacement at 2/32". Most dealers will try to sell you a new tire at 3/32". Stay legal and comfortable.
For me, I DO NOT like "unknowns". Why be going down the highway on your HD wondering about the tire? Also, when you get a new one verify that it is the proper load rating.
When is it time to replace my rear tire? I don't know how many miles are on these tires. The bike was used when I got it. Anybody?
Insert a penny in the tire tread. Abe's head first. The top of his head is 2/32 .
At 2/32 there should also be moulded "bars" in the tread.
They will show up when you wear down to that tread depth.