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Originally Posted by akhbhaat
While the numbers would indicate that BMW's cars run in the realm of 90-95% efficiency, this is due to a problem with the numbers, and not a magically efficient drivetrain.
BMW, Lexus, and everybody else use the same basic drivetrain configuration (to such an extent that almost all of them subcontract the production and even the design of components such as transmissions), and most of the losses are due to friction within the mechanical portions of the drivetrain itself. Some companies have slightly more efficient drivetrains, though the use of more efficient alternators and other parasitic devices (like the electric water pump in the E90), but it's not so significant that it could account for 10% of a difference or more.
The bottom line is that virtually all manufacturers underrate the stated output of their engines to some extent for a number of reasons. BMW in particular has been known to significantly underrate - if a given car or model appears to have an unusually small drivetrain loss (a very efficient RWD drivetrain will lose 15%), it is almost certainly due to the fact that the engine is making more power at the crank than the manufacturer claims.
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Thanks for pointing that out to me.