[namirez]

This is a common misconception. Flying at higher altitudes doesn't mean just lower air resistance; it means lower lift as well. As a result you have to fly faster for compensating lower air density. If you do the math, you realize the overall effect would be higher energy consumption.

[freepor]

Come again? The drag is proportional to the lift, but if you’re flying higher, and thus faster, to get the same lift, you’re getting to your destination faster.

[namirez]

What you're missing is the effect of speed and altitude on engine performance. Please take a look at this (pdf):

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.167...