With regard to the law, you are mistaken. The Church teaches that we are to submit to all legitimate civil authority – that is to say, we are to obey laws that are not unjust. Refusing to do so is therefore contrary to Church teaching.
Now, whether it is mortally sinful would depend on a number of factors, I’m sure – for example, how much one is speeding, whether one is being reckless in addition to speeding, etc. To merely say, however, that speeding is no big deal, is to flout the law, plain and simple. As Catholics, we have a duty to obey the law – period.
NB: I struggle with a habit of speeding, too – it’s remarkably easy to do when a) one is late, and b) a good portion of one’s drive to work is over a nice, long, straight bridge with 4 lanes in each direction…
Peace,
Dante