No, driving over the speed limit is not a mortal sin. It would not be considered serious matter in the moral life. That being said, if the lives of others or your own life is intentionally or unintentionally (while intoxicated, for instance) put in danger by the choice to drive recklessly, than it would be considered a serious venial sin or a mortal sin depending on the circumstances.
But, driving over the speed limit on a stretch of road does not constitute of itself a mortal sin.
I hope that this helps clarify it. Drive safely!