The m/c drivetrain may function just fine but it won't be the ultimate for fuel efficiency. If a guy was in a position to do an extensive development campaign he could re-cam and re-tune the intake tracts and exhaust system for peak torque and efficiency at lower rpms.
I haven't looked into it, but perhaps there are some motorcycle-like drivetrains that are more tuned for lower rpm torque: thumpers, dirt bikes, ATVs?
V Twins aren't necessarily all that torquey and efficient... at least my 500 SilverWing isn't. 45mpg is the norm, maybe 50 on a good day and yes it's all tuned up.
That reminds me of another point: motorcycle drivetrains appear to me to be quite lossy. By that I mean, unlike in most cars where the power goes from engine to trans input shaft to output shaft to differential to wheels, bikes usually (always?) employ chains or belts between engine and trans, and after trans too.
More chains and belts and shafts and **** means more drivetrain losses.