completely depends on terrain... I noticed it kicks into EV mode when on FLAT roads and DECLINES (of course)... then it will switch over to the engine when it detects the slightest incline or hill.
Not to really nitpick but the terminology is sort of wrong. I would be more accurate to say that the Niro will switch into EV mode more often when on horizontal roads any you are travelling under 60mpg. On a decline, it will go into the regenerative mode to recapture some of the energy. However, on an incline, the Nero can require the additional power from the engine to supplement the electric motor. As this is a hybrid type vehicle it can get power in one of the 4 modes. EV mode that is running purely on battery to drive the Niro using only the electric motor, ICE that used just the internal combustion engine to drive the wheels with any extra power directed to the electric motor in regenerative mode to recharge the battery, hybrid mode that used both the ICE and electric motor to drive the wheels, and regenerative mode that converts the momentum of the car directed to the electric motor in regenerative mode to recharge the battery and slow the car down.
The great fuel efficiency comes from the fact that any extra energy is captured rather than being wasted. So when the ICE is running, it is generally in the most economical rev range with the excess power going to recharge the battery. This is why if you look at the fuel economy bar on the dash as you drive it looks like you are using more energy than needed then decide to turn off the motor altogether. The engine by using a little bit more fuel to rev a bit more can move it into a more optimal zone of efficiency where that extra power can be converted without using up much more fuel, then when coasting that normally requires very little power from the engine can be switched over to the lower power electric motor that is far more efficient.