me old chap gets probably at best 22mpg on the motorway, and i darent think how low it gets at times, 16mpg wouldnt surprise me, from his Audi RS6, which is pretty appaling but what do you expect. it does have an 81 ltr fuel tank though.
The RS6 is awsome, i love the proper V8 rumble that you get, and the practicality. Its ideal as you can take the family out etc, a good all rounder.
The E46 M3 he had before was very economical considering what it was, he could get upto 30mpg on the motorway and in the 20's for normal driving, and only less when really booting it. The M3 was great, but round the greasy roads around here it was difficult to get the power down, the rs6 has amazing traction and the handling is pretty amazing for such a large car too. On a dry road, in the right gear, id take the M3, but for evryday the RS6 is great!
I had a smirk on my face when one of his friends told him he'd bought a new car, a toyota prius, which was just around the same time he bought the RS6, not quite as economical then
Oh and i get about 32MPg out of my 1.8 focus on a good run, 50 ltr tank, do about 350 to a tank, do mostly short journeys so sometimes mostly in the 20s.