This is a quandary. Let's try to analyze this.
When accelerating or climbing a hill, 4th gear would almost always be more efficient, since the mixture is generally richer than when cruising on a level surface, and you are, for all practical purposes, "accelerating", even though you are climbing the hill at constant velocity.
HOWEVER: In a gasoline engine, volumetric efficiency is lowered by throttling, so I would venture to guess 5th gear. But, friction horsepower increases exponentially with piston speed, so it all may be a wash. Still, if more fuel is injected, along with the heavier pedal pressure, but vehicle speed remains the same, on the same stretch of road, traveling in the same direction, and under the same road and weather conditions, then efficiency is definitely lower in 5th gear than in 4th gear.
One way to tell is monitor intake manifold pressure (or vacuum) and engine RPM while *cruising*. Efficient throttling provides a lower manifold pressure (more vacuum), while acceleration or hill climbing requires a higher pressure (less vacuum), but volumetric efficiency is more important than throttling efficiency when load demand is high.
Note, this applies to gasoline engines. Diesel engines are a different story...
...I'd say climb the hill in 4th gear. 5th gear is for level freeways. :-)
-- -john wide-open at throttle dot info
~~~~~~~~ "The first step in intelligent tinkering is to save all the parts." - Aldo Leopold ~~~~~~~~
