2006 BMW 335i SE - Should I?

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How do you find the 330i in terms of day to day running costs, the 335i claims an average of around 29mpg do BMW tend to be optimistic with their figures? I guess it's possible to check the cars average mpg through it's life with the idrive.. (Still not sure if this has idrive).

I've owned a 2006 manual 335i SE coupe for nearly 3 years and long term I'm getting around 28mpg.
Most I've had on a motorway run has been 38mpg like Fox and regularly get 35mpg on the motorway