God wants us to obey the law of the land. However, if there was a serious emergency would it be a sin to go way over the speed limit (therefore breaking the law) in order to solve that issue?
It depends on what the emergency is. Is there a life at stake? Or would it simply be convenient to get somewhere faster? The latter case certainly would not constitute an emergency. By serious emergency I presume you mean that there is a life at stake. I also presume that you are referring to a situation that could not be better solved by calling for emergency assistance and waiting for emergency personnel to come to you (because, of course, calling for emergency assistance should be a first resort and driving for assistance a last resort). In such a situation as that, not only could you morally go over the speed limit – as long as you were doing your best to maintain a reasonable level of safety – but if you were pulled over, in a life-or-death emergency a police officer would either assist you at the scene or escort you to an emergency room with sirens.