Minumum Sink Rate/Best L/D at 17,000 feet ?

Having done most of my flying at lower altitudes, I have wondered
about the contradicton between my unscientific observations when
flying at high altitude and what I would expect from my somewhat
limited knowledge of physics and aerodymanics. I certainly believe
that true airspeed increases with altitude. I use a rule of thumb of
about 2 percent per thousand. So (at 17,000 feet) a Indicated
airspeed of 42 knots becomes 56 knots true airspeed. An indicated
airspeed of 70 knots becomes 94 knots true airspeed. It just does not
feel like or the instruments don't seem to indicate sink rates (I have
made no careful observations) one would expect for the higher true air
speeds. Is there no free lunch?