What’s the tacit speed limit on a 70 mph highway?

Back when the max speed limit on highways was 55 mph, there was a sort of tacit understanding between cops and travelers—the roads can handle higher speeds, so moderate speeding will be overlooked. As I remember it, this usually meant you were in no danger of getting pulled over for running at 65. On the New York State thruway north of Rockland County, the tacit speed limit was more like 75. That was a great road, and people flew down it.

Now, over many parts of the big interstates, the speed limit is 70 mph. I don’t do much speeding anymore, at least not intentionally. Driving is much more pleasant at normal speeds. However, when the road is straight and empty, it’s pretty easy to creep up to 80 without noticing.

So, what I’m wondering is, has the tacit speed limit crept up incrementally with the written speed limit, or does 70 mean 70? Has anyone gotten a ticket at 75 mph in a 70? 80? Please advise. My tiny grant budget won’t withstand a $200 traffic violation.