At high altitude because the air is thinner and the temperature is as you say much lower, the indicated air speed(IAS) is much lower that the true air speed(TAS). The TAS is the indicator of your speed over the ground depending which way the wind is blowing of course.

Because the drag is lower for the TAS since the IAS or pressure air speed is so much lower, the fuel flow and thrust needed to maintain that speed is up to half of what would be required at sea level.

The basic answer then is that you need about half the fuel to maintain a specific groundspeed at high level than at sea level. The engines also operate more efficiently as well because you need higher RPM to maintain the thrust required and thus the engines are operating at an RPM where the maximum compressor and turbine combined efficiencies are at their greatest.

Les :)