My two-penneth worth...
Had my CX-5 (150 PS diesel manual) three weeks tomorrow and have about 940 miles on the clock. I've been averaging an indicated 44mpg. I haven't had a chance to calculate manually from fill ups and mileage etc but plan to do this now like others suggest.
I could get 45mpg (indicated) on a run with careful driving in my old Golf 1.6 petrol so i'm thinking this average isn't remarkable so far (but obviously the engine needs to loosen up yet). I travelled from SE London/Kent border to Plymouth and back in the CX-5 and still got an indicated 44mpg average but the journey involved delays both ways (typical for travelling to the West Country in August!)
So, wanting to reassure myself that better mpg figures are actually possible, I reset the fuel economy meter on leaving Fleet services on the return journey. From there to the M25 are roadworks with an average 50mph speed limit. With cruise control set to 50, some 18 miles later. At the junction with the M25, I had an indicated mpg figure of 87.5! (I got a picture as evidence!). On getting home, having travelled a further 55 miles or so round the M25, again using cruise and going no faster than 65, I had an indicated mpg figure of 63.6 (I took a picture of that too!)
OK, so this is in no way scientific or particularly realistic but it's a least given me hope that mpg figures of 50+ should be possible in future, on a run in the right conditions!