Owned a petrol BMW over a decade ago that was more economical when using the cruise control.
Conversely, ran a courtesy car Golf Bluemotion for a week last year, and it proved significantly more thirsty than my right foot for identical engine mode, journey types and pace on dual carriageways (though the irritating radar element - not a fan - I'm sure didn't help economy). This was all driving the Bluemotion with a curiosity to see how frugal it could be.
I guess it is all down to the manufacturers' different algorithms.
Cruise control
Re: Cruise control
Came back today. Averaged 31mpg which is maybe 3 or 4 better than normal. 