While gliding the C-150 (fixed pitch, 48" climb prop, and standard O-200) at idle, I consistantly obtained a VSI rate of descent of 450 FPM trimmed at 60 MPH.
...
With the mixture leaned, and the prop stopped, I glided - oh what a peaceful sound! The observed descent rate was exactly the same, at 450FPM. Again, I allowed it to settle for more than a minute at 60 MPH, and the result was consistant.
I can't tell from what you write what exactly you're comparing with. There are three possible states:
1) Prop turning, fuel burning, with throttle set to the idle stop
2) Prop turning, no fuel being burnt
3) Prop stopped, no fuel being burnt
From what you write, it sounds like you're comparing 1 and 3. The engine still produces substantial power at idle, so you're trading off the power of the engine against the work of turning the prop.
Regardless, I think the effect of stopping a fixed pitch prop is much less than the effect of feathering a variable pitch prop. There was a NACA paper on this -- I'll try to dig it out.