I own a BMW e46 330 petrol convertible, is a 07 335diesel alot more economical?

I own a petrol 330 ci m sport convertible, love the car but need something more economal but still has power. Looking at the 335d m sport coupe. I currently get 21 mpg on my on board computer, but need something that will deliver around 30mpg. Hears the 335 diesel gives about 27 or 31 from various websites.. Mixed figures tho? Any feed back would bs great in regards to fuel economy, fuel tank size ( is it large or small fuel tank), road tax price and general feedback. I own 02 330 petrol, looking at a 07 335 d m sport