Well, for a start the roadster is heavier than the coupe so there's one or two MPG down the drain. Next, the CD is considerably higher, it's 0.34 compared to the coupe at 0.30, at motorway speeds say 75 that's got to be at least another 2 MPG, maybe more? That obviously doesn't account for all the missing MPG but I know Caroline's roadster only gets an overall average of 21.1 (and I do around 50% of the driving in that car) compared to my coupe's 26.1.
Pete