IMO what your complaining about is what your asking for.
You want a realistic gear box, ok, but that is not a gearbox from some lightweight sports car.
Those are TONS you have rolling down that hill. Drop it into any gear but the exact correct one with the right amount of throttle and you should have an exploded gearbox, broken drive train or no tracks, or both plus tank tumbling down the hill.
Just for a rough example, try it in a farm truck with several tons of grain onboard, rolling 30 mph, and drop it into low and drop the clutch. Risks FAR FAR outweigh any benefit. Just was not done with tanks I do not believe. For sure not at more than a couple MPH.