A320 FLC REDUCE AIRSPD 7 KTS DURING CRUISE TO LESSEN PAX BUMPY RIDE DUE TO TURB. ARTCC CTLR LATER INTERVENED AND ASKED AIRSPD AND WAS ANNOYED THAT THE CREW HAD NOT ADVISED OF THEIR REDUCTION.

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WHILE IN CRUISE WBOUND AT FL350 NEAR RAPID CITY; SD; WORKING WITH ZDV; WE WERE IN CONDITIONS OF 'CONTINUOUS LIGHT CHOP.' PER STANDARD PROC FOR PAX COMFORT; WE REDUCED OUR AIRSPD A SMALL AMOUNT TO PROVIDE A SMOOTHER RIDE. KNOWING THAT AN AIRSPD CHANGE OF NO MORE THAN 0.02 MACH/10 KTS IS WITHIN LIMITS ON AIRWAYS ABOVE FL240; OUR SPD WAS ADJUSTED TO 0.77 MACH/262 KIAS FROM OUR EARLIER SPD OF 0.79 MACH/269 KIAS. SINCE NO RPT IS REQUIRED TO ATC; WE DID NOT ADVISE ZDV OF OUR AIRSPD. ZDV ASKED US OUR AIRSPD (MACH). WE REPLIED '0.77.' HE ASKED THE ACFT BEHIND US WHAT THEIRS WAS. I DID NOT MAKE NOTE OF THEIR REPLY; BUT ZDV DID 'GRIPE' AT US THAT WE SHOULD TELL HIM WHEN WE SLOW DOWN. ACCORDING TO THE RULES OF IFR FLYING; OUR SPD REDUCTION DID NOT REQUIRE A RPT; SO WE JUST ANSWERED 'ROGER.' JUST AS WE WERE SWITCHED; WE WERE TOLD TO CONTACT THE DEN SUPVR. OUR DISPATCHER INTERVENED AT SOME POINT DOWN THE LINE AND TOLD US TO FORGET CALLING AS HE HAD STRAIGHTENED IT OUT. ON LNDG IN PSP; HOWEVER; I CALLED THAT NUMBER AND LEFT A MESSAGE ON THE ANSWERING MACHINE THERE. I PRESUME IT IS NOW 'CASE CLOSED.' WE KNOW THE IMPACT THAT UNEXPLAINED SPD EXCURSIONS CAN HAVE ON TFC SEPARATION AND ALSO KNOW THE STATED EXPECTATIONS OF ATC. OUR SPD VARIATION WAS WITHIN LIMITS AND NO RPT WAS REQUIRED.