Atleast in my car, 72 is the mark on the speedo where the car is supposed to shift into 4th gear
So putting it in 'S' at that speed, would bring the RPM's up super high but you are just gunning it anyway to get there...should take your foot out of it :-/
I accidetnally downshifted the other day (trying to slow down coming over the top of a hill and down the other side)
doing about 80-85..put it in 'S' and freaked out as I realized what speed I was going and looking at the tach (poor engine)..
if you are having problems going up a hill, and your car is doing decent rpms...then you either are missing power somewhere...or that is one nasty hill..
your situation sounds like my 300SD when the banjo was plugged..

besides..
the car has more torque down lower in the RPM range (right?)