I think that's correct. The relationship of Volts <-> RPMs is based on the number of turns or windings in the motor. The RC folks often talk about the "KV" of a motor, which (i think) basically means how many RPMs per Volt you get, (which is directly proportional to the turns or windings). If you check out http://goldenmotor.com/SMF/index.php?topic=4492.0, they theorize that the Golden Motor has a KV of 83.3 (which works out to 2000rpm for 24V, 4000RPM for 48V, etc).
Sometimes, a manufacturer can wind the motor to have a different KV. I know that Alexey at Revolt can do that (he asked what KV I wanted when I bought my motor recently), and I think this is how there are different versions of the ACXX motors within the same family (e.g. AC-20, AC23, etc).
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