Trawling around on the London Forums today and found this. It's only a very small download and seems to be reasonably accurate.
http://forums1.avsim.net/index.php?showtopic=254535I tried it on my current flight YPAD-NZCH using the PMDG 738, and after inputting a total of 125 passengers, a flight distance of 1728nm, FL350 with a headwind component of 41kts, information gathered from Active Sky, it seems to have calculated a fairly accurate fuel requirement of 33.4 or 76% capacity, as well as a flight time of four hours and ten minutes.
As well it also calculated a ZFW of 120.55 with a fuel reserve of 5.3, which was marginally less than what I had worked out using my trusty calculator. Also the second page gives you a loading diagram, indicating centre tank at 56% with left and right tanks at 100% each, and the third page a printable (if you really need it) page with all, and I mean all, of the relevant information for the flight.
It can be used to calculate all of the 737NG aircraft, as well as the B744. It seems to be a little more conservative than the PMDG config. editor and at this stage certainly worth a try for those that are interested.
Regards,
Gary