Ramen Noodles is spot on.
The calculations he sets out are exactly the same calculations that are done for glider performance (albeit with a much better glide ratio). The curve is called a polar, and can be calculated for any aircraft that can glide. As a general rule:
- If you are in a head wind or a sinking airmass, then you should increase your airspeed to get the best range over the ground.
- If you are in a tail wind or a lifting airmass, then you should decrease your airspeed to get the best range over the ground.
Daniel