Originally Posted by feta_mgp
I recall just fine how they were rated and I'm sure that if you DROVE in SelectShift all the time your mileage would suffer, justifying the lower number. However the gearing and transmission are the same... adding a feature that allows the driver more precise control isn't going to cause any difference unless the feature is used. Don't use SelectShift, don't affect mileage.
If you only used the automatic sport mode all the time your mileage could suffer due to locking the car out of 6th gear, but if you know how to drive a manual and shift gears already your mileage would actually improve or at worst stay the same. However per the test conditions they had to shift through certain gears at a given time which is counter to the car being left in automatic mode alone.