i have a cdti 150 astra and its only more economical in 6th gear if your travelling above 85mph on a motorway. the engine labours far to much below 60 in 6th. no way on this earth its economical unless your travelling above 85 and on a motorway. get about 45 mpg pottering about 50+ on a decent journey. my e46 M3 averaged 34 mpg+ on the trip to germany and back.