I grew up quite litterally a few feet away from route 66, middle-of-nowhere. I remember when they raised the speed limit from 55 to 65. But, it's only a two-lane with no median. Also, it winds around a lot.
Now, if you're talking interstate highways then it's usually 70. In oklahoma there is no "night" speed limit but in Texas the night speed limit is 65. I like to hit the corners at over 80 at night, and near 90 during the day. Cruising I usually hang around 82 if I'm being paranoid about getting a ticket, if I'm not being paranoid then I'll bring it up to just under 90. However, when I get into the city (and when I say "the city" I mean the whole Denton-Dallas-Fort Worth area-
the darker area on this map; zoom out to "6", I travel on I-35 from oklahoma all the way to the dot in the middle of dallas) it goes down to 65 and eventually to 60 as I get further in. Those areas I usually see at least 3 cops so I feel it's dangerous to go over about 75 or even 70 in some areas.
Yesterday, though, it was really funny. I was in the middle of Dallas headed south and a cop pulls onto the interstate. Of course, everyone immediately slows down to the speed limit. It's about 5 lanes of solid traffic all going the speed limit, following the cop because noone dares pass him. Of course, about a quarter mile up people are going normal speed and they keep getting further and further so basically you have this vast empty space and then a solid wall of traffic. I was lucky and happened to be in front so I didn't have anyone blocking me in. What do I do? "Cya, losers!" *jams pedal down* and cruises right past the cop at 75mph in a 65 zone. It was really funny to see all of those paranoid people going the speed limit as I just zoom right ahead and the cop does nothing. Of course, if someone else had done it I'm sure they would have gotten pulled over. I'm special, I get to speed right past cops all the time and not worry about it. But, then, that might be because I have a military tag