SPD DEV BY A B767 FLC BECAUSE OF LIGHT CHOP IS CAUSE FOR ATC TO LECTURE THE CREW REGARDING TFC SEPARATION AND SPD REDUCTIONS THAT REQUIRE AN ATC NOTIFICATION; 172 MI E OF GRB; WI.

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IN CRUISE AT FL310; MACH .81; WE ENTERED AN AREA OF CONTINUOUS LIGHT CHOP AND INTERMITTENT LIGHT TURB. I ASKED THE FO TO SEAT THE PAX AND ADVISE ATC OF THE RIDE. AT THE SAME TIME I SLOWED TO TURB AIRSPD OF MACH .78; WE ALSO SENT AN ACARS MESSAGE TO DISPATCH ABOUT THE RIDES. HOWEVER; WE OVERLOOKED ADVISING ATC OF OUR SPD REDUCTION. AFTER WE HAD BEEN SLOW FOR ABOUT 2 MINS; HE ASKED IF WE HAD SLOWED; AND THE FO REPLIED THAT WE HAD. THE CTLR WAS LESS THAN HAPPY WITH US BECAUSE HE MUST HAVE HAD ANOTHER COMPANY ACFT CLOSE BEHIND US AT THE SAME FLT LEVEL. THIS WAS NOT INTENTIONAL ON OUR PART; BUT WE DID GET BUSY WITH TAKING CARE OF THE ACFT AND OUR PAX. I DID CHK TO SEE IF WE HAD SLOWED MORE THAN 5% OF OUR MACH NUMBER AND TRUE AIRSPD WHICH WENT FROM 481 KTS TO 463 KTS AND WE DID NOT. ACCORDING TO OUR FOM; A SPD CHANGE OF 5% OR MORE REQUIRES A RPT TO ATC. IF THIS IS INCORRECT; PLEASE LET ME KNOW. I DO FEEL WE SHOULD HAVE LET ATC KNOW OF OUR SPD CHANGE EVEN IF IT WAS LESS THAN THE 5%. I WILL IN THE FUTURE MAKE SURE AS SOON AS I MAKE A SPD CHANGE OF ANY KIND ON MY OWN; NO MATTER HOW SMALL; TO LET ATC KNOW. THIS OCCURRED APPROX 150-200 NM E OF GRB.