Wonder what other W220 S500's get on a motorway cruise. I am getting around 28mpg with cruise control (Distronic actually) on. Car is 11 years old but has been well maintained. I think that is pretty reasonable.
All these figures seem pretty good when you consider the size of these engines - work colleague was telling me about a friend of his who has a 6.6 V8 Firebird Transam from the 70s and he gets at best, 9mpg and usually 6mpg.