I tried searching but I'm not good at it! Optimal tire pressure for longest mileage? 2018 ultra limited. Stock tires.

 

Spec is 36F/40R, with Dunlops I see much better wear on my customers bikes setting them at 38F/42R...The bigger part of the equation is checking them often as under inflated tires will get hot and wear quicker...

FWIW, I run my Avons at 40-42F and 44-46R. Different tires like different pressures...

 

Whatever HD recommends. Sure, pump them up for better mileage, and you’ll have a dime sized contact patch. The best tire pressure is a compromise between ride quality, handling, traction, tire life, and others. The best to know that are the HD engineers, so go with what HD recommends. Keep checking your pressure and balance.

Anyone that says use what’s on the sidewalls is grossly uninformed.

 

I check air daily. I have FOBO TPMS. I have been running 40 and 42 but wasn't sure what has been the popular settings? Naturally when asking a dealer they will only tell you what's in the book.

 

I've found the recommended tire pressures to be much higher than my own personal "compromise" numbers. I start there, then lower pressures until I get what I want.
On my bike at least...(1985 FXRS) I haven't noticed any significant loss of mileage on my tires.