There is a theory, which has supposedly been proved with some careful testing, ( sorry, no link ) that says the optimum, is enough pressure to result in a 15% deflection of the sidewall when loaded. If not racing, any more pressure results in a harsher ride, and possibly losses from bouncing, with little gain from reduced rolling resistance. I try to follow this now, and while it does seem a tiny bit slower, the ride is very nice on Supremes at 60/65 Front/Rear.

Of course, if racing, then seconds count.