I contend yes. Not a mortal sin (probably not a mortal sin usually) but a venial sin. For the following reasons:
1. Thou shalt not kill. Someone (supposedly educated professionals) has determined a safe speed in a given area. To go above that means you are likely adding additional danger to your safety (and others with you). (I suppose if you truly believe a speed limit is too low, the proper course of action would be to try to raise the limit, rather than speed.)
2. Honor thy father and mother. This commandment extends to being obedient to legitimate civil authority (such as police officers and laws). We are obligated to obey civil laws unless immoral. Again, if we disagree with those laws, we can try to change them.
3. Thou shalt not bear false witness. Not sure if this is the right commandment here, but speeding causes scandal. If I am speeding and my 16 yo daughter is with me, I am setting an example that it is okay to disobey (at least some) civil laws. If I see a priest (Roman collar) zooming down the freeway at 90 mph in a 70 mph zone, he is saying that we do not need to obey speed laws.
Even 1 mile over the limit is speeding. That may arguably be less serious than 20 miles over the limit, but it is still over.
For the record, I usually speed. Not by much, maybe 5 miles, seldom more than 10 over. I think it is wrong (for the above reasons), I do it anyway. Something for me to work on.
Also, I do believe there ARE times when speeding is NOT sinful. Two examples come to mind- the true emergency; and also in certain traffic situations. Certain times on certain freeways, if you drove the speed limit, you would be MORE of a hazard and danger than if you sped some. If everyone is zipping along at 80-90 and you're putzing at the posted limit of 65, you're a hazard. In that case, I would say speeding (as minimally as safe) is the lesser of two evils and not sinful.