How about instead of just keeping track of the total speed, keep track of the actual propulsion speed 'generated' by the the tank itself?
In real life a car can also go faster downhill than it can on a flat surface... When going down hill, you will hit your normal 'maximum' speed and still be able to accelerate more.
In real life a car can also go faster downhill than it can on a flat surface... When going down hill, you will hit your normal 'maximum' speed and still be able to accelerate more.