I've noticed the Prius V engine is running at very low RPM in ECO mode at steady speed of about 65km/h (40mph). Fuel consumption vary between about 4-5L/100km (58 - 47mpg) and RPM is showing about 1100+ I usually see RPM of about 1100 to 1400+ which is very good; so I'm not surprised this engine is very fuel efficient.
The Power Split Device allows the Prius to only generate as much torque as needed to push the car down the road at the speed you select. When that requires little torque, you get very low RPMs compared to other automatics. (or even manuals with few gears) Once wind speed demands more power to maintain, RPMs go up more than linearly with speed.
