Heat is the big enemy of tire life. Vehicle manufacturers' recommended pressures are a compromise of comfort and handling characteristics. When you run a tire at lower pressures, the sidewalls flex more and generate heat causing decomposition of the rubber over time. Running more pressure but not more than the tire makers' recommendations on the sidewall will generally give a longer tread life at the expense of a comfortable ride because the sidewalls are not flexing as much as the tire rolls. I run 40F & 42R and got 16,000 from my OEM's and could have got more. I changed to Avons running the same pressures and now have about 4,000 on them with no noticeable wear. Of course aggressive riding and road surface also has an effect.
After the OEM's on my Valkyrie puked out at 7500 miles(I was running Honda's recommended PSI) I started running 40F-42R in the following 6 sets of tires(various brands)and always got 20,000 miles from them.