With modern lubricants it may not make any differnce or at least it may be a very slight difference but just going to extreme comparisons we'll think about this a bit.
Possibly during acceleration some effeciency cold be gaind with an engine always working in it's most effecient RPM by always changing from one ratio to the next most effecient range but at crusing speed or an extreme comparison being no transmission at all just a direct hookup from the engine to the rear differential. No transmission at all would cause no drag so no loss in horsepowr being consumed by friction. The second extreme being a six or however many speed transmisssion with the torque being transmitted thru it, there being all sorts of tbings turning inside plus the drag of the lubricant in the trasmission case all consuming a bit of the torque along the way to the differential. So I guess what I'm getting around to say is that likely an auto spends 99.9% of its road time in a direct drive situation while also spinning a bunch of other things in the tranny that are doing nothiong but producing drag. Yes, I realize this is not a likely situation having no transmission being a regular gasoline or diesel engine would struggle to get the car moving from a dead stop.

Frank.

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