The engine efficiency will not be; rolling resistance will not be; wind resistance certainly is not; ...

I suspect university labs have done these tests and spent great time and money doing so just because the result comes from multifactoral parameters. Car companies will definitely know this data but would never release it.

This does not invalidate the question - I recall it is best to get up to speed as fast as possible. But is that right? Drag racers would then be the most efficient - at least at startoff!

An old VW Polo or Golf had a vacuum at the carb, somehow, linked to a dashboard dial showing "MPG" at each moment. You don't see these now; why?
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muymalestado