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???

Metal tracks add a lot weight to the tanks...generally between 25-50% of the original weight. So, yes, I think it could be the addition of the metal tracks.

The weight added with metal tracks for the most part is un-sprung weight they will have a tendency to sit the nose and you will notice that the front drops more as a result of the metal tracks the reduction in speed will be caused by more by the extra weight being transferred through to the motors this can be corrected by replacing the motors with slightly larger versions however I find that the reduction in speed as a result of the tracks is more satisfying and realistic.

Thanks guys.
I did put in bigger motors (long).
I like the slow speed, but what speeds are typical for tank wars?

Weight is a bit deceiving. It is really the pulled inertia required to track metal over the wheels as opposed to plastic.

I am surprised your speed dropped by about half, my experience shows only about a 20% reduction.