Actually, states like NY, NJ, PA all seem to have pretty low limits on the interstates. Max 65mph and it goes down to 55mph in urban areas.
On the other hand, the country road limits are pretty high outside cities, usually 55mph, but can go down drastically in cities.
But other states, I think mostly in the south and west do have higher limits.
Yeah, most of the northern states still keep it at 65. I drove from Toronto to Florida a couple weeks ago, and the only states that I drove through that had a 65 rural speed limit were NY, PA, and MI. Personally I loved driving through the states where it was 70mph. It allowed me to do 120 km/h without any worry of being pulled over.
The only time where it seemed to drop to 55 in urban areas was where it was in really urban areas, with a lot of entrance and exit ramps (through downtowns for example). Through most of the suburban areas, it was 65.