These days I use the posted speed limit on my GPS as a guide. I tend to keep at about 8 miles over. I never exceed 10 miles over. I drive a company car so I cannot get any traffic violations. If I get more than 2 in a 1 year span, I'm screwed. Plus, that seems about right to me. There are certainly times when the posted speed limit seems just right to me. In particularly, when driving through the mountains, it can get really steep and curvy.
Most municipalities can ticket you for 8mph over. 6 they can but they rarely do.
-There's definitely an element of knowing where and when to take your chances, right? For example, there are areas of Ohio where I wouldn't do more than 3 mph over the posted limit of 60mph. They notoriously ticket out of state plates. My disdain for the Buckeye state knows no bounds.