Hi mate,
I run a 2000 MY S8 on LPG, for my normal driving on A and B roads I see about 200-220 miles from £26-£28 worth of LPG. This equates to arounr 18mpg.
Let loose on the motorway and plodding along I can get up to 26mpg, tried it, achieved it, got fed up with caravans overtaking me...