Sin is not relative to whether we "feel" like we're sinning or to whether we determine the effect it might have on others. Nor should we comparing ourselves to others behavior to determine the morality of an action. If perfection is God and we use pure white as His color, and pure evil is Satan and we use pure black as his color. Why would comparing yourself to gray make you pure white? It only makes you think your gray is better than their gray. We should be comparing our gray to God's pure white and striving for that.
I'm not a police officer and I don't pretend to be but the signs say, Maximum speed XX and Minimum speed XX. In Maine we also have signs that say, Slow down in inclement weather. These are the laws that are in effect to keep people safe. My understanding is that they are established based on crash tests, engineering assessments of roadways and prudence on behalf of the department of transportation and the state lawmakers.
With those things in mind I don't see how anyone can look at going over the maximum speed limit as anything less than breaking the local law. I won't say that I don't do it either but I know it's not right and I certainly wouldn't tell a police officer that he can't fine me because I was only Xmph over the maximum. This is a confession item for me all too often.
As to it's gravity, we go back to the three conditions for a sin to be mortal:
SOGFPP said:
"Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."
Is it a grave matter? Well if the law of the land thinks that the maximum safe speed on the road is 65 because over that more accidents or more severe accidents occur, then I'd say it's grave matter.
Do you know you are speeding? If you look at the posted limit and you look at your speedometer, then I'd say you have full knowledge.
Are you deliberately consenting to sin? If you know you are over the posted limit and you continue to speed then my guess is yes you are deliberate consenting to sin.
Obviously, not knowing the speed limit or not being aware of your own speed would move it from mortal to venial.
If I'm off base here someone stop me but it seems logical to me.