WRONG TKOF DATA USED FOR WT; GO NO GO COMPUTATIONS.

Narrative

WE HAD JUST ARRIVED AT THE ARPT AND LANDED ON THE E RWY. AFTER DEPLANING AND ENPLANING PAX; WE WERE SET TO GO. THE ATIS HAD RPTED CLR SKIES; 25 MI VISIBILITY; A TEMP OF 98 DEG AND SOUTHERLY WINDS; 180 DEG 10 KTS. I HAD PLANNED FOR A RWY 15 DEP AND THE ATOGS; ACTUAL TKOF GROSS WT WERE WELL WITHIN LIMITS. DURING PUSHBACK AND ENG START; THE CAPT REMARKED THAT HE'D LIKE TO DEPART ON RWY 26. WE HAD DONE THAT VERY THING THE EVENING BEFORE AND THE OP WORKED BEAUTIFULLY. HOWEVER THAT HAD BEEN LATE AT NIGHT AND THERE WASN'T MUCH TFC. THIS WAS EARLY EVENING AND IT WAS BUSY AND I NEVER DREAMED THAT TWR WOULD APPROVE A WESTERLY DEP; HENCE; NO PLANNED OR ANTICIPATED RWY 26 DEP; WHEN HE REMARKED A SECOND TIME THAT HE'D LIKE A RWY 26 DEP; I ASKED GND IF A RWY 26 DEP WAS FEASIBLE. MUCH TO OUR SURPRISE; THE GND CTLR SAID 'YES' AND SWITCHED US TO TWR. I HAD JUST COMPLETED THE AFTER START CHKLIST WHEN TWR CLRED US ON TO THE RWY AND CLRED US FOR TKOF IMMEDIATELY. THE DISTANCE FROM OUR PUSHED BACK POS TO THE TKOF SPOT WAS ABOUT 500 FT; SO WE EXPEDITIOUSLY COMPLETED THE PRE TKOF CHKLIST BOTH ABOVE AND BELOW THE LINE. ALTHOUGH IT WAS NOT MY LEG; I STILL MANAGED A QUICK LOOK AT THE IMPACT OF THE NEW RWY AND ITS EFFECT ON OUR DEP; BOTH NORMAL AND IN THE EVENT OF AN ENG FAILURE. I ALSO NOTED AND SET THE NEW CHANGED DEP FREQ INTO THE 'ON-DECK' SELECTOR AND FINALLY; IN RESPONSE TO THE CAPT'S QUERY; QUICKLY CHKED THE ATOG FIGURES TO ASCERTAIN THAT WE WERE INDEED LEGAL TO TKOF. (WE HAD A SOUTHERLY WIND WHICH GAVE US A HEADWIND COMPONENT WHICH CAUSED THE CAPT TO CONSIDER THE RWY IN THE FIRST PLACE.) BY THIS TIME; THE PWR WAS COMING UP AND WE WERE ON OUR WAY. AFTER TKOF AND DURING THE CLBOUT; I BEGAN TO RECHK OUR NUMBERS AND I DISCOVERED THAT WE HAD EXCEEDED THE RWY ATOGW FOR THE NEW RWY. I WAS APPALLED AT MY UTTER FAILURE TO READ THE ATOG CHART CORRECTLY; IN MY HASTE TO COMPLETE ALL MY TASKS (PUSH BACK TO AIRBORNE TOOK 6 MINS). I READ THE WRONG COLUMN WHICH CAUSED ME TO THINK WE WERE WITHIN ATOG LIMITS WHEN; IN FACT; WE WERE NOT. PREVENTION 1) BETTER ADVANCE PREPARATION 2) A CHANGE IN PLANS SHOULD WARRANT A SECOND LOOK AT EVERYTHING 3) SLOW DOWN. SUPPLEMENTAL INFO FROM ACN 314806: FO WAS ASKED IF WE 'COULD DO IT' MEANING; ARE WE LEGAL AND UNDER THE GROSS TKOF WT TO USE THE RWY. THE FO LOOKED UP THE DATA (I PERSONALLY SAW HIM USE THE PROPER PERFORMANCE PAGE) AND ANNOUNCED 'YES.' THE BOTTOM LINE IN THE MATTER IS THE FO JUST MESSED UP. ALL PROCS WERE FOLLOWED CORRECTLY. DUE DILIGENCE WITH NORMAL DOUBLE CHKS. NO SHORTCUTS WERE MADE OF ANY KIND. THIS MISTAKE WOULD HAVE RESULTED IN AN OVERRUN OF THE RWY ONLY IF THE ABSOLUTE WORSE CASE ACCELERATE STOP ABORT SIT OCCURRED. NO DANGER WAS INVOLVED IN A SINGLE ENG TKOF (GO) LET ALONE NORMAL 2 ENG TKOF.