Is fuel consumption less at high altitude?

No - if it is being combusted with oxygen to produce power. There is less oxygen in the atmosphere the higher you ascend, hence producing less power. Therefore to produce a comparable power output, you need to use more fuel. Witness the power drop you feel in your vehicle when driving at high altitudes plus increase fuel consumption, and the performance drop athletes experience when competing e.g. Olympic games in Mexico which is 7000 ft above sea level.