Hello.
Unfortunately, the information on the 20D motor does not indicate the gearbox that is attached to it, but there are other ways we could potentially identify that. Can you measure the length of the gearbox? Checking that against the information in our 20D gearmotor dimension diagram should narrow down the possibilities.
From there, I recommend running your motor without any load attached, measuring its free run speed, and comparing that to our free run specs for each gear ratio. A tachometer would be the ideal tool if you have one, but you could probably just manually count the number of rotations the output shaft completes in a minute. If the output turns too fast for you to count, you can run it at a lower voltage and then scale it to determine the free run speed at the nominal voltage. For example, a motor that runs 600rpm at 12V should run 100rpm at 2V (12/2=600/100) , which should be much easier to count.
- Patrick