It'd be nice if those of us who are serious about phonographic disk and
cylinder recordings were to CEASE using the word "speed" for the rate at
which a disk or cylinder turns. Speed (velocity) is distance divided by
time. (i.e. miles per hour, inches per second) "RPM" is not a speed. Also,
the turning rate should not be referred to revolutions per minute, because
they don't revolve: they ROTATE. Notwithstanding the early (referring to
the manufacturers of machines and records) lingual mangling of "speed",
"RPM" in geometrical and mechanical fact means Rotations Per Minute except
for those disks in which the speed remained constant & the rotational rate
varied. Because the speeds of a preponderance of disks (consumer, ETs,
film soundtrack) continually change, it's best to refer to their RPM's,
whether the user etymologically discriminates between "rotation" and
"revolution" or not.
i.e. Assuming a 16" disk turning @ 33 1/3 has its outer most grooves 3/16"
from the edge & 5" from the center, its speed range would be determined by a
maximum dia. of 15 5/8" (15.635") and minimum of 5". Its speed range would
be 27.27 down to 8.72 ips!
i.e. Edison DD 52200's (one of my favorite Happiness Boys disks) outer most
grooves have a dia. of 9.25" & inner most of 6". Thusly its speed is 38.72
ips down to 25.12.
Note that cylinders also rotate, they don't revolve. Warping
notwithstanding, given that their playing diameters don't change, their
speeds are constant. A 2" cylinder cut @ 120 RPM has a speed of 12.56 ips.
At 144: 15.07 ips. At 160: 16.75 ips. Assuming that a Concert cylinder's
playing surface is actually 4", then at 120 RPM those puppies run @ 25.12
Now let's think about those damned ubiquitous "Drive Slow" signs....