I've been running those gear boxes with the " Graupner speed 400 (7.2V) for almost 3 weeks in my pershing ....
I'm quite happy with the results , as it did rise the top speed and gave me enough torque to mow the lawn in the park ...
I bought those gear boxes 5 years ago from " RC Command) and they"ve been at least in 3 different tanks since ...They are indestructible ...
Note that I'm also running my pershing with the plastic tracks ...