major, thanks for the charts - they are very helpful.

Does the inefficiency also reduce the torque, or just the output power?

The first chart, running at 4000 rpm with 90 Nm of torque requires about 38 kW. Let's say this is an electric Smart car. With a tire circumference of 1.1 m and a reduction ratio of 7:1, you get 37 kph. The efficiency is 92%. At 35 kW power, you will be accelerating quite a bit. So you accelerate to 74 kph and the rpm is 8000. Your efficiency is about 93%. Now you are approaching 111 kph which is 12,000 rpm. From the chart you see that the efficiency is quickly being pushed down to 80, 70, and then 60%. The faster you go, efficiency drops dramatically.

What is causing this sudden drop in efficiency?

It goes to show that depending on the motor and load conditions, the motor can become so inefficient that if we just increase the power without accelerating, we will actually be using less power, per unit of work.

BTW. my calculation for RPM was amiss. Karter2 has the correct speed.