Easiest for me is to think about fuel consumption per hp. Most recips burn in the region of 0.45-0.5lbs/hp.
In my aircraft with 260 hp a side, 520 total, this means about 42gal/hr knuckles to the wall at 26in MP. At 18000 ft the output has gone down to about 45% and the total hp available is now just 260-270. Therefore your consumption is now around 21-23gal/hr. You also gain about 2kts of TAS for each 1000ft in altitude, so it's easy to see that cruising high is almost always better, even in a moderate headwind. Obviously one has to take into the account the time to climb and consumption to get there - it might not be worth it on shorter trips. But on most longer trips it is.
A power reduction at high altitude decreases consumption further, so if you're not in a hurry you can cover long distances at low fuel flow. In fact, if you were to regularly cruise at the aircrafts max ceiling at best glide speed IAS (least amount if drag), that's when you'd save the most.
Last edited by AdamFrisch; 27th October 2012 at 21:34.