Noticed one thing on today's flight. I was flying behind one aircraft online on VATSIM for a while, and was talking to the guy. He reported that his FL was 370 and I was 390. And we both had altimeter set to 1013 Standard. So I guess, TCAS should report -2000ft difference. But instead, it reported -2400ft difference.Why?

Air Data Computer (ADIRU) calibration errors? +/-200' in each aircraft is probably well within real world tolerances at that (high) altitude.What was the other guy flying? In cruise, some aircraft don't fly exactly at what the Captain's and F/O's instruments show.... due to "inertial" smoothing....but, I guess, being a sim, you would expect a perfect world... not a real world? :-hah Cheers.Ian.

Being a sim, I wouldn't expect such sort of thing to be modelled.I was flying the route to Venice and he was flying to Malta. It so happened that he was at the FL370 and I wanted to climb to FL390, and that we just met from two airways, and basically flown the same speed and I was just a little behind him, then I overtook him as I was below and then again manually slowed down to come behind and climb over him. And I noticed that when I was passing FL370, I was leveled with him, which would suggest we were at the same altitude. Too bad I can't remember what TCAS said when I was at FL370 exactly.

You probably had different weathers loaded in your sims, so the barometric pressure differences were somewhat off. That's my guess anyway...