Is there a way to turn off EV mode?

Hmmm. I thought I observed it higher. I will need to check with my son after his trip this weekend. (2015 Prius v)

 

Different car though. Maybe the v is higher?

I encounter a steep downhill grade with a 60 kmh speed limit, and just beyond bottom of the hill the limit goes to 80. The car will accelerate without giving gas, and engine always kicks in around 73 kmh.

 

Just chipping in to relate post of my journey today. I had a high SOC and had to descend a long steep hill behind a reeeelly slow car so I went into B mode. The slower move turned off at the bottom and I motor up the other side of the valley at up to 35mph with a full SOC and all the motive power coming from the battery but no EV indication lit up and the ICE never started up at all.

 

Exactly.

When you look at the efficiency diagram of the internal combustion engine (ICE), there comes a point where no matter what transmission and no matter how you choose the control parameters, below a certain amount of power output, the ICE becomes much less efficient than the electric motor. And that is including the losses from generator --> converter --> battery charging --> battery discharging --> inverter --> motor.

Even at highway speeds, a slight downhill or a slight deceleration can reduce the required power into the range where electric power is more efficient. At that point there are two options: a) switch to electric power, or b) use the generator to increase the load on the ICE, thereby bringing ICE into a more efficient part of the power range.

The latter option, sometimes called load leveling or load shifting, is sometimes counterintuitive to some hybrid beginners. The best time to recharge the battery is not when the ICE is nearly idling, but rather when the ICE is producing a moderate amount of power, close to its point of maximum efficiency. Then not only is the ICE more efficiently propelling the vehicle directly, it is also using less gasoline per kilowatt hour to recharge the battery.

 

Two more things to consider:

Firstly, chronological age is also a factor in battery aging. Even if you hardly use the battery at all, it will still slowly degrade with time. So take advantage of it before it dies of old age.

Secondly, the Prius is not designed to minimize fuel consumption and emissions at any cost, but to do so cost effectively. For every dollar that you "lose" because the Prius is "wearing out" the battery, you gain two (not exactly, but you get the idea) dollars in fuel and maintenance savings.