Intuitively, I would say yes, though I have nothing scientific to back up that statement nor any idea of just how much slower would be optimum...
91040 wrote:If 38 mph is the sweet spot for a flat road, does that sweet spot become a lower speed as the grade increases?
Notice the chart has rpm and torque on the 2 axis's. Extreme torque and rpm conditions, both low and high, are less efficient, so optimum-range speed is probably quite a bit lower than 38 uphill, and higher than 38 downhill.
Using climate control (energy used is a function of drive time) will also increase the optimum-range speed, and higher drag conditions (windows open) will lower it.
The very broad 95% efficiency area in the middle of the graph probably reflects the experience we've all had, of getting consistently good range at speeds between 30-45 mph, as long as you can limit power use (no steep grades) to two "bubbles" or less.