I have been using your 20D metal gearmotors for some time now on a 4-wheel robot, and I was considering an upgrade to your 25D motors. However, looking over the specs for the 20D vs the 25D, it appears that your 220RPM (34:1) 12V medium-power 25D motor produces 63 oz-in with a 2.1A stall current, but the equivalent 220RPM (63:1) 20D 12V long-life motor produces 64 oz-in with a 1.6A stall current.
So, it appears the 20D motors are smaller, lighter with the same RPM and torque as the 25D equivalent. Am I missing something?
Your assessment comparing those two motors is correct, though one thing you might not be considering is that the 25D gearmotors (since they are bigger) have larger and stronger gears that allow them to handle higher instantaneous torques. As long as you are keeping applied loads so that the motor operates around 25% of the stall current, that probably will not be an issue, but it could be a factor in applications where there might be rapid accelerations and decelerations (e.g. switching directions frequently at higher speeds).
Also, in case you are not already aware, we also have a series of high-power 12V 25D gearmotors that do produce more power than any of the 20D gearmotors, though they also have much higher current demands. The one with the most similar free-run speed to the 20D gearmotor you mentioned would probably be the 47:1 gearmotor, which has an extrapolated stall torque around 165 oz•in.
Thanks for the quick response. Glad to know I can still read technical specs, even at my advanced age ;-).