ACR ON DSCNT TO CLT; FOLLOWING FLT PATH OPTIMIZATION SYSTEM PROCS; WAS QUESTIONED BY ATC REGARDING SPEED REDUCTION.

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DSNDING FROM FL300 DOWN TO FL220 (ASSIGNED); CENTER CTLR ASKED OUR SPEED. CAPT REPLIED; '240.' CENTER; 'WHY ARE YOU DOING THAT?' CAPT; 'TO CONSERVE FUEL; JUST DOING WHAT OUR COMPANY TELLS US.' (IE FLT PATH OPTIMIZATION SYSTEM). CTLR; 'WELL; YOU DON'T DO WHAT YOUR COMPANY TELLS YOU; YOU DO WHAT I TELL YOU TO.' HIS REPLY WAS VERY NASTY IN NATURE THEN HE TOLD US TO MAINTAIN 300 KTS. WE COMPLIED. A FEW MINUTES LATER HE HANDED US OFF TO THE NEXT CTLR; 'IF YOUR COMPANY WILL LET YOU!' NO OTHER PROBLEMS OCCURRED. WE WERE NEVER ASSIGNED A SPEED UNTIL HE ANGRILY ASSIGNED 300 KTS. I WAS USING FLT PATH OPTIMIZATION SYSTEM DURING EVERY PHASE OF FLT. WHEN HE TOLD US TO DSND I FOUND THE SPEED FOR THE NEWLY ASSIGNED ALTITUDE AND WEIGHT. (NORMALLY DURING DSCNT I FLY THE CRUISE ALTITUDE MACH NUMBER UNTIL INDICATED BECOMES 290 KTS THEN FLY 290 THE REST OF THE WAY DOWN.) IF GIVEN AN INTERMEDIATE LEVEL OFF I WILL FLY THE NEW FLT PATH OPTIMIZATION SYSTEM SPEED FOR THAT ALTITUDE UNTIL WE ARE GIVEN ANOTHER DSCNT. FOR THE LEVEL OFF AT FL220; I NOTED THE NEW MACH NUMBER. (I MAY HAVE ACCIDENTALLY USED THE +40 TAILWIND NUMBER INSTEAD OF THE -40 HEADWIND THAT WE HAD.) A DIFFERENCE OF 267-240 KTS. EITHER WAY WE WOULD HAVE BEEN WAY BELOW 290 KTS USING FLT PATH OPTIMIZATION SYSTEM. AGAIN; WE HAD NOT RECEIVED ANY AIRSPEED RESTRICTIONS AT ANY TIME DURING CRUISE OR DSCNT PRIOR TO THIS EVENT. I WAS SHOCKED AT THE CTLR'S ATTITUDE AND CAN ONLY COME TO THE CONCLUSION THAT HE MUST HAVE THOUGHT HE ASSIGNED US AN AIRSPEED AND WE ACTIVELY DECIDED TO FLY FLT PATH OPTIMIZATION SYSTEM INSTEAD AND MOCK HIM DURING OUR REPLY AS TO WHY WE WERE GOING SLOW. BOTH THE CAPT AND I WERE PERPLEXED AS TO HIS ANGER. IT IS FOR THIS INCONSISTENCY THAT I AM FILING A RPT. THE CTLR WANTED US TO FOLLOW HIS DIRECTIONS BUT NEVER GAVE US ONE TO FOLLOW.