> a significant benefit from allowing the ICE to remain in the RPM "sweet spot" rather than moving around a larger range.

I remember buying plans from the Whole Earth News for such a car back around 1980. That was the selling point - keep the ICE running at an optimal point. I've not seen those same plans reproduced online.

Aha! https://store.motherearthnews.com/products/hybrid-car-bluepr...