Last time I checked, Mach numbers are not converted by how fast your going but I believe at or above FL230. I heard that on a ZNY sector one night. Anyway, the main reason for asking them is to make sure the sequencing on a particular jet/victor airways isn't gonna be ruined by improper/various speeds. I'm going to explain how they make sure their spacing should remain within standards. Generally, you'll hear it go like this. Let's say there are 4 guys level at FL220 going southwest bound on J75 into DCA, BWI and IAD.
(Comair 5100 is climbing out of kennedy and is cleared direct COPES to join J75 and he will be in the lead of this group)
"New York Center, Comair 5100, 17,300 for Flight level 220"
"Comair 5100, New York Center, Roger. What's your speed in the climb?"
"We're holding 300, Comair 5100"
"Comair 5100, Maintain 300 knots or greater, your leading the pack"
"300 or better, Comair 5100"
(Meanwhile American 226 is 15 miles in-trail and gaining
"American 226, What's your speed?"
"310, American 226"
"American 226, Maintain 300 knots"
"300 on the nose, American 226"
(Another aircraft, Jetblue 449 is also 15 in trail of the American)
"Jetblue 449, What's your indicated?"
"We're at normal speed, given to us by the last sector, Jetblue 449"
"Jetblue 449, Maintain 300 knots for in-trail spacing into Dulles"
"300 knots, Jetblue 449"
(And the last aircraft in this group, at the back of the pack is United 990 who is about 12 in-trail of the Jetblue)
"United 990, What's your speed?"
"310 for United 990"
"United 990, Maintain 300 knots or less"
If you understand, the guy at the front should maintain a speed or greater. The guys in the middle maintain the speed alone, no more and no less. The guy all the way in the back can maintain that speed or slower. I have heard ZNY do this with packs up to 15 aircraft long. So this is a somewhat short example. Sorry for the essay.