Like others, I suspect, I've struggled to find good data on the true Kv (RPM/V) for the BPM motor. The vendors seem to advertise confusing, meaningless or just plain wrong data, expressed as "RPM at 36V", often without saying whether this is no load or loaded. I did manage to find a table from Suzhou Bafang that gives data for some motor winds (code numbers) and I have used this to derive estimated Kv values for the codes that we don't have data for, using a derived motor constant based on a linear interpolation.
Bafang quote an RPM tolerance on their data, and I've repeated that here, with an estimate of the RPM tolerance they might use for each. This gives a maximum and minimum RPM tolerance either side of the nominal value for a given voltage that could expected due to manufacturing tolerances.
The RPM for any voltage and motor wind can be estimated from the data below by just multiplying the value of Kv for a particular motor by the battery voltage. For example, a Code 13 motor running on 49 V could have a no load speed of anywhere between 314 and 338 rpm, with an average motor being around 326 rpm at this voltage.
Please ask questions on the forum, rather than by PM, as it helps others and you'll get a better range of answers.