A CL65 CREW INSTRUCTED TO MAINTAIN 170 KTS SPD TO THE OM SLOW TO APCH SPD TO AVOID OVERTAKING AN ATR72 4 MI AHEAD.

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DURING OUR APCH CLRNC TO RWY 36L; WE WERE INSTRUCTED TO MAINTAIN 170 KTS TO THE MARKER. AT THAT POINT; WE WERE TOLD THAT WE WERE APPROX 4 MI IN-TRAIL OF AN ATR72. BEING IN AN RJ; THE CAPT WAS AFRAID OF OVERTAKING THE SLOWER PROP ACFT. WE WERE APPROX 4-5 DME PRIOR TO THE MARKER WHEN HE STARTED CONFIGURING AND SLOWING. VREF WAS AROUND 138 KTS; AND WE WERE MAINTAINING 140 KTS; 30 KTS SLOWER THAN ASSIGNED. AROUND THIS POINT; ATC (CVG TWR) QUERIED US AS TO WHAT SPD WE WERE MAINTAINING. WHEN I TOLD THEM; THEY ASKED WHAT SPD WE WERE ASSIGNED. WE WERE THEN TOLD TO SLOW TO APCH SPD. WHEN WE LANDED; WE WERE TOLD TO CONTACT THE APCH CTLR. IN A DISCUSSION WITH THE CAPT AFTER HE CALLED; HE RESTATED HIS CONCERN ABOUT THE SPACING BEHIND THE ATR; AND COMMENTED THAT 'COMMAND AUTH' GAVE HIM OVERRIDE AUTH OVER ATC COMMANDS. WHAT HE HADN'T KNOWN; BUT WAS EXPLAINED TO HIM BY THE CTLR WAS THAT RWY 36L AT CVG HAD LOWER THAN STANDARD SEPARATION REQUIREMENTS BECAUSE OF MORE HIGH-SPD TURNOFFS. SUPPLEMENTAL INFO FROM ACN 561388: THE APCH CTLR CLRED OUR FLT FOR THE APCH TO RWY 36L AND TO MAINTAIN 170 KTS TO THE MARKER. AT THAT TIME WE WERE APPROX 5 MI BEHIND ANOTHER ACFT. APCH THEN HAD US CLRED FOR THE APCH AND TO CALL TWR. BUT NOW THE ACFT IN FRONT WAS STARTING TO SLOW DOWN AND WHEN HE WAS 3 MI AHEAD ON THE TCASII; I ALSO STARTED TO SLOW DOWN; HOWEVER JUST OUTSIDE THE OM. THE TWR ASKED US WHAT OUR AIRSPD WAS. WHEN I TOLD THEM 160 KTS THEY ADVISED ME THAT I WAS ASSIGNED 170 KTS TO THE MARKER AND THAT THEY EXPECTED ME TO HOLD THAT SPD UNTIL PASSING THE OM; OR SHORT FINAL. AFTER BREAKING OUT OF THE OVCST WE NOTICED THE ACFT IN FRONT OF US STILL ON THE RWY. BUT BY THE TIME WE CROSSED THE THRESHOLD THE ACFT HAD CLRED AND WE LANDED. AFTER CLRING THE RWY TWR ASKED ME TO CALL THEM. WHEN I CALLED THE TWR SUPVR; HE INFORMED ME THAT APCH RADAR CAN SEPARATE US WITH LESS THAN 3 MI. ALSO; WHEN THEY ASSIGNED A SPD THEY EXPECTED US TO FLY THAT SPD. HE TOLD ME NOT TO USE MY JUDGEMENT IN ACFT SEPARATION; AS THEY HAVE APCH RADAR AND TWR PEOPLE THAT WILL MAKE THAT CALL FOR US. IF WE ARE GETTING TOO CLOSE AND NEED TO BREAK OFF THE APCH OR TO MAKE A GAR; IN CASE THE ACFT IN FRONT HAD NOT CLRED THE RWY IN TIME FOR US TO LAND; THEN ATC WOULD INITIATE THE G-A.