Tank guns have such high velocities and are so accurate (at least NATO tanks) that they are practically line-of-sight weapons. If you can see it, you can destroy it. So there generally would be no instance where a modern TD using direct fire cannons can out-range a tank.
My understanding is that American requirement is more intended as a fire support vehicle than a dedicated anti-tank platform.
[This message has been edited by buzz_knox (edited September 08, 2000).]