Residential: (Limit 35 or under) +5
Main Road: (limit 40 to 60) +10
Expressway: (limit 55) +25
Tollway: (limit 55) varies by road, but usually +30
It is very dependent upon the road. There are some towns that will write tickets at 1 MPH over and some expressways where traffic usually moves at 90mph in the mid lanes or nearing 100 in the fast lanes depending on traffic. My current car is limited to 81, and it will not go fast enough to be in the fast lane on some expressways; especially 290 or the extensions where the slow lane traffic is usually above 80 (limit 55). When I used to have a much longer commute many years ago, I would often get passed by folks doing 95+ in a 55, and the police would stick their arm out the window and point down to have them slow down, but wouldn't ticket them. I know a few state troopers in my state (helps when a friend's dad owns a bunch of convenience stores, so cops are there always for free breakfast) and there are no hard and fast rules. Most of the ones I talked to said they would ticket faster on bad driving (lane control, dangerous driving, etc) than on speed alone; but going a lot faster than traffic would get you stopped.
Every single one also said being polite is the best way to not get a ticket when pulled over. They don't want to get out of their cars to talk to you on an expressway. For example, many moons ago I was pulled over by a state trooper when I was driving well over a hundred in my Ram (no traffic at all, middle of the night on expressway). He yelled at me, I was polite, apologized, and he knocked my ticket down to 84, which is the maximum allowed for taking the online traffic school class so the ticket doesn't go on your record.