The efficiency is poor at low motor speed, which is expected. For most efficient operation, you should start in the lowest gear which can be used for any significant time, and shift up only when the resulting motor speed after the shift is at least 1500 RPM. 3rd and 4th gears would be appropriate... but only at high enough speed. This might be hard to do, for anyone accustomed to maximizing engine efficiency by keeping the engine speed as low as possible.
To really understand what motor speed to use, performance data over the range of loads would be required - the data provided by NetGain is only for full load. More complete efficiency maps are published for many motors, including some version of the same motor family as the HyPer 9. The map in the NetGain sales sheet for the HyPer 9 came from SME, and is (deliberately) without a power scale so actual values are not available, but it does show that even at moderate loads the ideal motor speed is not the lowest possible; optimal efficiency is at roughly 1/3 load and 2,000 RPM, and efficiency dives rapidly as speed drops below about 1200 RPM at any load.
Regardless of efficiency, the HyPer 9 is only rated for something like 3,300 RPM continuously - something else to keep in mind when choosing gearing. In the efficiency map, the HyPer 9 favours operation below the midpoint of its speed range, which seems unfortunate for a vehicle with a fixed gear ratio: if you gear to run 6,000 RPM at 100 km/h (60 mph), then you're out of the peak efficiency range when over 50 km/h (30 mph) - which means most of the time for most drivers - and you're over the continuous speed rating as soon as you get off of a residential street.
The Nissan Leaf map is widely published. The best point at full power is around 6,0000 RPM (more than halfway up that motor's speed range); at lower power levels the optimal speed rises with power, and would be as low as 1,500 RPM... but only for about 4 kW.