Accelerating up to 40 MPH under electric motor power alone is very difficult and it does not use the battery energy to advantage. The ICE is programmed to operate under acceleration conditions in combination with the electric motors. If you accelerate excruciatingly slow you can probably reach above 30 mph if all conditions are just right. The engine is most efficient under a load and moderate acceleration is one of the efficient modes for the ICE. High speed driving with the consequential wind resistance and driving up hill are other efficient modes for the ICE.

If you travel 45 mph and decelerate to just below 40 MPH with your foot off the accelerator, the engine will stop if the vehicle is warmed up and it is not unduly cold outside. You can now get the vehicle to run on electric for a short duration only as long as the terrain is flat - no hills or even a slight incline -and you do not try to accelerate.