Sorry, I don't mean 'true' EV mode (via a switch,) I mean just plain 'running on electric only'. From what I understand, when in the true EV mode, the ICE kicks in if you exceed 35 mph. (or maybe it's at 35.) In 'running on electric only', (i.e. not EV by switch, just driving carefully,) I've gone as fast as 62 mph on a slight downhill on the freeway and the display has said that only battery power was going to the wheels. But, again from what I understand, the ICE will at least spin without gas when you exceed 42mph, because the electric motor can't handle going any faster than that, so the ICE would need to take up some of the rotational energy.
On level terrain, I have gotten up to 40 on electric power alone. (Also, the lower the battery charge, the more likely it is to use the ICE. On one or two bars, it's almost impossible to make the car move forward on battery power alone, even downhill.)