A320 FLC DECLARES MINIMUM FUEL ARR ORD.

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ON DEP PLANNING AT MEXICO CITY; FORECAST WX ENRTE SCATTERED RAINSHOWERS AND ARR AT ORD 3800 FT BROKEN; 8 SM; FAR MINIMUM 4.0 AND 3.0 CONTINGENCY/HOLD FUEL. NORMAL FLT TILL CORPUS CHRISTI; TX; GIVEN 300 MI RTE DEV TO LLO; COWBOY (DALLAS) THEN REJOIN FLT PLAN RTE. THE FUEL PREDICT COMPUTER SHOWED 2300 LBS AT ORD. WE CONTACTED OUR DISPATCHER IMMEDIATELY; AS WE WERE LOADING IN THE NEW FLT PLAN. NO REASON GIVEN BY ATC FOR RERTE. ABOUT 20 MINS LATER; GIVEN ANOTHER RERTE -- THIS TIME DIRECT DFW THEN FLT PLAN RTE. THE ERROR BY ATC CAUSED A 2000 LB FUEL LOSS. NOW FUEL PREDICT COMPUTER SHOWING 4.9/5.1 AT ORD. DISPATCH ALERTED; GAVE US MIDWAY AS AN ALTERNATE AND TOLD US 5-10 MIN DELAYS INTO ORD. AS WE APCHED BDF; WE HEARD HOLDING INSTRUCTIONS FOR OTHER ACFT 30 MINS TO 1 HR. AGAIN; CONTACTED OUR DISPATCHER WHO TOLD US: 1) WE WOULD GET PRIORITY HANDLING; AND 2) MIDWAY WAS A GOOD ALTERNATE. WHEN OUR HOLDING CLRNC WAS ISSUED; WE SAID 'WE CANNOT ACCEPT THE HOLD.' ATC ASKED IF WE WERE DECLARING 'MINIMUM FUEL.' WE SAID 'YES.' PROCEEDED DIRECTLY TO ORD TILL NEXT CTLR TRIED TO VECTOR US 90 DEGS OFF COURSE TO R. WE DECLARED 'MINIMUM FUEL.' WERE PUT IN LINE FOR 25 MI FINAL AT ORD. ARRIVED AT GATE 4400 LBS (FAR MINIMUM 4000 LBS). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THAT NO REASON WAS GIVEN FOR THE ENRTE ROUTING CHANGES. HE ALSO INDICATED THAT THE COMPANY'S FUEL MODEL NEEDED UPDATING; AND WAS JUST RECENTLY CHANGED. THE CREW WAS DEBATING WHETHER TO DECLARE AN EMER; BUT APCH GAVE THEM THE ADVISE OF DECLARING 'MINIMUM FUEL' FOR EXPEDITIOUS HANDLING.