G280 captain reported an issue with ATC in regards to understanding best forward speed.

Attributes
ACN	1429721
Time
Date	201703
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Gulfstream G280
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	STAR
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Flight Engineer Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 72 Flight Crew Total 10521 Flight Crew Type 1082
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Deviation - Speed All Types

Narrative:

We were descending in ZZZ. ATC asked our speed; and I replied '320 knots; but we can give you more.' the controller asked for 'best forward speed;' and I replied that we would comply. We were later assigned a descent to cross zzzzz intersection at 12;000. We responded and began the descent at the appropriate time. In the G280; vmo decreases from about 340 down to 300 KIAS as you descend from the low 20's to 10;000. We maintained best forward speed throughout our descent; but the controller asked if we were still doing 320 as we neared zzzzz intersection. I responded that we had slowed; but we're still giving him best forward speed. He told us to let him know before we slow. I believe that he vectored an aircraft off the arrival; behind us; but do not believe any separation was lost. In this case; I didn't connect his desire for us to maintain 320 knots; with what I understood to be 'best forward speed.' in the future; I will be more careful accepting an ambiguous airspeed clearance; knowing that we have so much variation in vmo at lower altitudes.

Original NASA ASRS Text

Title: G280 Captain reported an issue with ATC in regards to understanding best forward speed.

Narrative: We were descending in ZZZ. ATC asked our speed; and I replied '320 knots; but we can give you more.' The controller asked for 'best forward speed;' and I replied that we would comply. We were later assigned a descent to cross ZZZZZ Intersection at 12;000. We responded and began the descent at the appropriate time. In the G280; VMO decreases from about 340 down to 300 KIAS as you descend from the low 20's to 10;000. We maintained best forward speed throughout our descent; but the controller asked if we were still doing 320 as we neared ZZZZZ Intersection. I responded that we had slowed; but we're still giving him best forward speed. He told us to let him know before we slow. I believe that he vectored an aircraft off the arrival; behind us; but do not believe any separation was lost. In this case; I didn't connect his desire for us to maintain 320 knots; with what I understood to be 'best forward speed.' In the future; I will be more careful accepting an ambiguous airspeed clearance; knowing that we have so much variation in VMO at lower altitudes.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.