I recall years back seeing a video where a Prius was driven at 10/10 around a track, and a some expensive sports car (V8 Ferrari?) was driven right behind it. The Ferrari got better fuel economy.
Reason being, the 4 cylinder Prius engine was running way outside its efficient range, whereas the Ferrari's was just about dead center in its, driving at what was probably closer to 4/10 for that vehicle.
Mercedes have experimented with fuel efficiency in their Formula cars, using turbines in the exhaust to recover heat energy into electricity. I believe their thermal efficiency approached 50%.
https://www.roadandtrack.com/motorsp...ne-efficiency/
It helped in racing because they spent more time doing laps and less time refueling, even if the equipment to bring up the efficiency slowed it down.