I dropped my wife off at the airport this morning in her ML320 CDI and on the return journey my intrinsic curiosity got the better of me and felt I had to find out what sort of economy I could get from a now fully run in 4x4. I’m no ‘hypermiler’ so my speed was only limited by traffic conditions and national speed limits. I do admit to using a great deal of forward observation and the fundamental laws of conservation of energy. The air con was on and traffic conditions didn’t really warrant the use of cruise control. But there was also a following wind! The more loaded (!) and congested out bound trip was 8mpg less that this. The pleasant result leads me to conclude that aerodynamics are important in everyday driving and that I was right to lower my S Class in pursuit of improved efficiency. The overall figures for that car year on year have risen from 40 to 43mpg. I also feel Mercedes have done a good job with the efficiency of the latest four wheel drive system (as in ML, GL and R). An Audi Quattro from 10 years ago would take a 5mpg bite out of the economy of any similar engined two wheel drive model. They may not be very PC, but there is no reason why a large Mercedes 4x4 should be regarded as an ecological disaster unless perhaps it’s an ML63!