On my last flight a couple of weeks ago on a B747-400 from KJFK to EGLL I noticed that at 35000ft, with a 56knot tail wind (and I assume we were at M.85) it was indicating a ground speed of just over 600 knots. When I tried to replicate the flight in the the sim. it gave me a GS of 546 knots. Are there other components that I should consider, like out side air temp. ( in the sim it was -23c) Just curious as to why I can't get the same results.Regards, Russell.

OAT and pressure are 2 big things in calculating ground speed or mach. Since you were at 35,000ft your altimiter should be set to 29.92, so the pressure factor should of been the same on both flights. If the OAT was the same, your GS would be the same as before. If the OAT is lower then the GS would be lower, but you would be at the same mach. So, lower OAT would mean higher groundspeed, but mach would be the same.Seems like the recent flight, the weather up there was a bit colder than it was a few weeks ago.I written a program on my TI-83+ that calculates GS and mach from IAS, it seems like what I said is right LOL.

Thanks for that Zack, I thought it maybe to do with the OAT. I was a bit sleepy on the flight, and can't for the life of me remember what the actual OAT was. Much appreciate your comments, Regards,Russell.

Could be amount of fuel on plane as well naturally :)Or OAT in conjunction like fuel in reality changes mass and weight according to temperatures whereas i dont think FS got the sim THAT much real yet :D

Get your drift Cargomun, just too many variables which you can't get as a passenger (especially when you're half asleep) Thanks for your comments.Regards,Russell.