You never know when you might come across a cop who's having a bad day; dropped his doughnut(s), spilled his coffee, had a fight with his significant other.
I try to drive about the speed limit.
In the bad old days of the 55 mph limit, I was driving a company van at 60 and was pulled over by a highway patrolman.
He clocked me at 68, wrote me a ticket.
Turns out that when the van needed new tires, they put a bigger set on it for better gas mileage, which also had the effect of changing the speedometer.
I had they speedo checked at a local shop and it was 68 at an indicated 60. (The guy did ask, "what do you want it to read?")
I figured the truth would be good enough and get me off.
Took the day off from work, drove all the way from Raleigh to Asheboro to present the evidence and get the ticket dropped.
Judge says, "you said you thought you were going 60, that's still speeding. Pay the man".
You never know.