ACCEPTING AN OFFER FROM THE TWR TO DEPART RWY 25L INSTEAD OF RWY 25R; AS PLANNED. TO PREVENT FURTHER DELAY A B737-700 CREW DOES NOT RE-COMPUTE THEIR PERFORMANCE. ONCE AIRBORNE THEY DO RE-COMPUTE FINDING THEY HAVE EXCEEDED THE TAILWIND LIMIT FOR TKOF.

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PREPARING TO DEPART LAS; I PUT ALL ACTIVE RWYS AND CURRENT ATIS INTO THE OPC. ATIS WAS CALLING LIGHT WINDS; WITH RWYS 19 AND 25 IN USE; WELL BELOW MAX ATOG FOR BOTH RWYS 25R AND 25L. ATIS CALLED LOW LEVEL WINDSHEAR ADVISORY; SO THAT BOX WAS ACTIVE IN THE OPC. ON TAXI OUT; GND CALLED NEW WINDS FOR US; 130 DEGS/12 KTS. I PUT THOSE WINDS INTO THE OPC; DESELECTING ALL RWYS EXCEPT THE ONE WE WERE TAXIING TO RWY 25R; SO THAT THE OPC COULD QUICKLY RECALCULATE. NO PROBS NOTED WITH THESE WINDS. NO CHANGE IN V-SPDS (125-15-130); STILL MAX N1 (DUE TO 'LLWS' BOX). WE WERE CLRED INTO POS AND HOLD ON RWY 25R. LONG DELAY ENSUED DUE TO DEPS AND ARRS ON RWYS 19L AND 19R. TWR CALLED WINDS 130 DEGS/12 KTS GUSTING 24 KTS; THEN OFFERED US A RWY 25L DEP. CAPT ANSWERED 'YES' AND WE WERE CLRED POS AND HOLD ON RWY 25L. I PULLED OUT THE OPC; BUT REALIZED THAT THE TIME IT WOULD TAKE FOR IT TO CALCULATE EVERYTHING; AFTER I WOULD BACK OUT TO ENTER THE NEW WINDS AND NEW RWY; WOULD BE EXCESSIVE. I RATIONALIZE THAT 1) WE WERE ALREADY USING MAX N1; 2)WE HAD 6000-7000 FT STOPPING MARGIN ON RWY 25R; 3) THE FLT PATH IS SIMILAR FOR THE 2 RWYS AND 4) WE'RE IN A FAIRLY LIGHT B-700. ('WE HAVE GREAT PERFORMANCE!'); SO IT'LL WORK FINE. BOTTOM LINE; THOUGH; WE HURRIED; AND I DIDN'T HAVE THE CAPT WAIT FOR ME TO RUN THE NUMBERS. AT CRUISE ALT; I DID RUN THE NUMBERS; AND DISCOVERED THAT THE LAST WINDS TWR HAD GIVEN US HAD A 12 KT TAILWIND GUST COMPONENT. ALSO REALIZED WE HADN'T CHANGED THE DEP RWY IN THE FMC TO RWY 25L. I'M CONFIDENT I'D HAVE PUT EVERYTHING IN IF IT HAD BEEN A 'BIG' CHANGE (IE; TO RWY 19); BUT I MISSED 2 KEY ACTIONS (FMC AND OPC) WHEN IT WAS A 'SMALL' (RWYS 25L TO RWY 25R) CHANGE. LESSONS LEARNED: 1) EVERY RWY CHANGE IS A 'RED FLAG' EVENT -- DON'T RUSH THROUGH IT! 2) PULL OUT THE CHART WITH WIND COMPONENTS TO CHK FOR TAIL/XWINDS OUT OF LIMITS -- MUCH QUICKER THAN THE OPC! 3) WHEN LAS IS USING RWY 19'S AS WELL AS RWY 25'S; THEY MAY ASSIGN RWY 25L SINCE IT DOESN'T INTERSECT THE RWYS 19'S AS RWY 25 DOES. ALWAYS KEEP RWY 25L IN THE OPC. 4) DON'T RUSH!