For a piston engine (which one can call a
power producing engine) the interesting speeds can be derived from the power required curve. For a turbo-jet engine (a
thrust producer) you would use the thrust required curve or, to call it by a different name, the total drag curve.
Both curves have similar though different inverted U shapes and use TAS along the x-axis. They use different units for the y axis (power and drag, naturally).
You can see examples of these curves at:
http://www.allstar.fiu.edu/AERO/BA-Form&gra.htm
They are near the bottom of the page after all the frightening looking maths equations!
The curve of speed against power required has two interesting points:
1. The bottom of the curve shows the speed where power required is at a minimum. This speed will give minimum fuel flow i.e. max endurance. It will also give minimum sink speed for gliding and be very close to the speed for best climb angle.
2. The tangent to the curve from the origin will show the speed where the ratio of speed to power is at a maximum. Now, as power is equal to drag times speed, it can be shown by clever mathematics that this tangent also defines the minimum drag speed, or best L/D speed. That speed will give the best range, the best gliding angle and be close to the speed for best climb rate.
The curve is for a particular airframe and propeller configuration at a given weight.
So, for a Cessna 152, if the speed for best gliding angle is at 60kts, that speed will give the best L/D ratio for the aircraft gliding with the propeller windmilling. If the propeller is producing thrust, the L/D ratio will be improved at 60kts and you can fly a little faster before the drag and power required increases to the point where the ratio of speed to power is once again at a maximum. In other words, you have drawn a new power required curve because you have changed the configuration from a draggy windmilling prop to a prop doing some useful work.
That is why the best L/D speed is different at different power settings.
So if Cessna state that the best gliding speed is 60kts, that doesn't necessarily mean that the best range speed is also 60kts. It will be faster. But it's true that the normal cruise speed of say, 90kts, will be faster than that for best range. We accept the compromise that for a slightly increased fuel consumption, we can get a quicker flight time.
If you are critically short of fuel, you will get better range in still air, or with a tailwind, if you slow down. The wind component will have a large effect on the chosen speed, such that in a strong headwind, you may be better off actually increasing speed!
For a turbo-jet, the interesting curve is that of speed against total drag, but that's not the subject of your question!