I dont have any formal data, but my Chevy volt seems to do better at 66/67mph on the highway. I think it mostly has to do with the way the gearing works.
Too slow and it is stuck working as a series hybrid, speed up a bit and the engine is able to clutch in to power the wheels directly and I can observe the throttle position start modulating below its maximum to keep vehicle speed constant.
I typically roll my eyes at these claims but occasionally they do exist. I can certainly believe it with a modified engine.
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