I use efficiency mode on the highway, generally with cruise control enabled. It is slow to engine brake on descents and tends to lose set speed on ascents, in order to increase fuel efficiency. I personally switch into sport mode on ascents in order to maintain the set speed, and will also switch to sport mode if I need to pass other traffic rapidly on two-way roads. I have not systematically compared Auto vs. Efficiency in these cruise control situations, but anecdotally probably eek out a couple extra mpg.