737 fuel consumption

Flying the 737 with FsPassenger, my fuel consumption is extremely high. The flight log says consumption from Sinapore to Phuket Thailand should be just over 7,800 lb, but when I arrive I've used over 18,000 lb.

Plane is clean, (flaps up, spoilers retracted, and gear up).

Altitude: 20,000 ft
Speed: 320 knots

I just got FSX and FsPassenger. I have flown several years with FS9 and never had this problem.

Thanks for your help, Glen