I have a Tamiya King Tiger. It has a 3:1 ratio reducer added to the gearbox.When first built, I timed and calculated the scale speed at 27mph.

I just noticed after a few weeks that the top speed is a lot slower. Timed it again and it tops out at scale 12-13mph. Checked all the Spektrum settings, etc - all OK. The big change though was going from plastic to metal tracks. Could that be causing so much speed reduction???