Thats normal as far as I know.
I seem to remember reading somewhere that when your car is warmed up, if you drive at 60mph, in 6th gear the psi should read 60 or a little below.
This seems to be true for my car, on a really long run on a hot day it may get as low as 50 psi.
This also works for 30mph in 3rd, 40mph in 4th, and 50mph in 5th - should all read about 50-60 psi