From a post on measurements made about a year ago on my Tamiya Tiger 1...I think I fixed the test voltage at 6V.

"I got my hands on a tachometer and measured motor speed and drive wheel RPM. Motor speed was fixed and averaged about 2000RPM (varied about 1900 -2100) and drive wheel was 20.5 RPM. Total gear reduction then is about 98:1.
The gearbox is a Tamiya with a Schumo "3x geardown" unit on it. Standard Tamiya motor."

The small size of the gearmotor gearhousings indicates small gears. Could they really handle the same loads of the much larger gears in the current gearboxes? Better materials maybe?
Good things besides their compact size is that they are very quiet and are sealed (no lube or dirt issues?)