Is speeding a sin? No. Just because speeding is a technical violation of the law and you may get ticketed for it does not mean that it's a sin.
"Everyone else does it" is actually a valid defense. America is a democracy in which sovereignty is held by the citizens and limited powers are delegated to the government. As a society we have reached a broad consensus that driving five to ten miles per hour above the posted speed limit, with the flow of traffic, is acceptable when conditions are safe. This consensus has more legitimacy than the traffic laws, and in fact the consensus depends on the posted speed limit being lower than the safe driving speed. We have a safety margin built into the speed limit, and we all agree that under good conditions it is safe to exceed the limit.
A few considerations:
- It is incumbent on us all to drive in a safe manner, regardless of the speed limit. Sometimes conditions are so bad that it isn't safe to drive the posted limit.
- When you exceed the posted speed limit you are accepting additional responsibility. If you cause an accident while speeding then this is strong evidence that you were driving too fast to be safe.
- A good rule of thumb is this: if twelve strangers (e.g., a jury) were to sit in judgement of your driving habits would they condemn you? If you're driving 40 MPH on an empty 35 MPH road on a clear day I think you'd be fine. If you're driving the speed limit while texting and weaving in and out of congested traffic in a construction zone you might be legally in the clear but I bet your peers would happily punish you.