Hm. I'm no physicist but I think your math is off. Remember it's velocity squared. Your car going that fast is has more than a kiloton of TNT or approaching 5 terajoules of energy... that can't be right.
For a 2 tonne car, my very rough math puts it at more like 2100 mph.
I can't edit my post now, but to be clear I was talking about kinetic energy. I would have thought momentum is not a very useful thing to talk about in a case like this where there was some pretty obvious transformation of energy into damage, etc.
As I said though, I'm far from a physicist so happy to be schooled if I'm way off...
For a 2 tonne car, my very rough math puts it at more like 2100 mph.