I did all planning for a long flight, considering JS41, and I was surprised that FMC showed me in the middle of flight I was 1h short. DTG was 465 and GS 306, then I still had 1:30 to reach destination, bt FMC was showing 2:32. 1h longer.I did the math an noticed I was ok and probably FMC wasn't doint its math properly. 4 minutes later it shows the correct ETE.I really don't know what was wrong, didn't change de FP and all my previous calculations done before flight used distance displayed at FMC. I is not boddering me but think you guys should check. I'll also monitor this time during my flight to see if it repeats.

See pg 112-113 in the AOM, which suggests you insert "A" in the trip plan field to update ETE and DIST... I've always gotten this info from the NAV pages.