I would say that the average POSTED highway/freeway speed in the US is between 55 - 65 MPH....However, most drivers are doing 70-75 MPH. If you drive any slower you'll cause problems and cops know this, so they leave you alone. Going 80-85+ for extended periods of time will get yo a_zz in trouble though...At least in the Philadelphia, PA Metro Area.

 

Cool, lucky you.
Anything over 109km/h (+-68 mph) will get you pulled over by the fuzz over here. They are pretty strict, so a radar/laser detector is a must over here.

 

Speed limit in Spain is 120kmh, which is about 75mph. But the cops wont bother you unless your doing over 90-95 mph. Even then they wont give you a ticket, just a warning, and put it in the computer. They'll only give you heavy fines when your going 40% above the speed limit, which would equate to about 105mph. But then again, in Easter it's not uncommon to get around 60 deaths in one weekend from accidents.