CPR JET REDUCED SPD WITHOUT INFORMING ATC. SPD DEV.

Narrative

WE WERE AT CRUISE (27000 FT) JUST BELOW REDLINE AIRSPD. CTR ASKED OUR AIRSPD. WE REPLIED 275 KIAS. CTR ASKED IF WE COULD INCREASE AIRSPD. WE REPLIED NEGATIVE. THEY DID NOT ASK US TO MAINTAIN ANY PARTICULAR AIRSPD NOR INDICATE A RELATIVE AIRSPD SUCH AS 'MAINTAIN BEST FORWARD SPD.' THEY THEN ASKED AN AIRLINE TO MAINTAIN 250 KTS FOR SPACING. THEY DID NOT INDICATE TO EITHER OF US WHO THE SPACING WAS FROM. ALTHOUGH WE ASSUMED IT WAS US. SOME TIME LATER; WE WERE GIVEN A DSCNT TO FL240. AS WE HAD BEEN JUST ON TOP AT FL270; THE DSCNT PUT US INTO THE CLOUDS AND SUBSEQUENT MODERATE TURB. THE PAX WERE UNABLE TO AVOID SPILLING THEIR DRINKS. I SLOWED THE ACFT TO 220 KTS AND STABILIZED THE DSCNT. WE THEN CHKED ON THE PAX. I WOULD ESTIMATE 1 MIN PASSED DURING THIS TIME. WE THEN ADVISED ATC WE HAD SLOWED TO 220 KTS. WE DID NOT HEAR ANY DISCUSSION ON THE FREQ; WHICH WOULD HAVE INDICATED A REDUCTION IN SPACING WAS OCCURRING. AFTER A MIN OR 2 FURTHER WE WERE GIVEN A PHONE NUMBER TO CALL UPON LNDG. DURING THE PHONE CALL WE WERE TOLD THERE HAD BEEN NO VIOLATION OF SEPARATION STANDARDS; BUT THAT WE SHOULD HAVE ADVISED ATC OF OUR SPD REDUCTION SOONER; AS THE AIRLINER WAS GAINING ON US AT 100-110 KTS. WE ASKED HOW THAT COULD BE IF HE WAS ASSIGNED 250 KTS (ONLY 30 KTS FASTER THAN WE WERE GOING). ATC REPLIED THAT THEY WERE DSNDING HIM ON TOP OF US AND HE PROBABLY COULDN'T SLOW DOWN THAT MUCH IN THE DSCNT. AT NO TIME ON THE RADIO DID WE HEAR THE AIRLINE CLAIM HE COULDN'T COMPLY WITH THE SPD RESTR. I GUESS IN THE FUTURE I WILL ADVISE ATC ON SPD CHANGES SOONER. BUT I WOULD ALSO EXPECT ATC TO ADVISE ME IF THEY NEED A CERTAIN SPD FOR SEPARATION. I WOULD ALSO EXPECT THE PLT OF THE OTHER ACFT TO ADVISE ATC IF HE IS UNABLE TO COMPLY WITH AN ASSIGNED AIRSPD.