65, except through urban areas, where it's even lower.
65
There are several Interstate Highways with a 75 MPH speed limit at various points throughout their span, but there isn't one which has that speed limit at every single point.
70
60
The speed limit on Interstate 90 in South Dakota is 80 mph or 128.7 km/h, except in city limits where the speed limit is lower, usually 55 or 65 mph.
50
scalar
As close to the posted speed limit as possible.
The highest speed limit I know of on an Interstate highway in the U.S. is 75 MPH, but that varies.
70 MPH on Interstate Highways.
70
A scalar, since no direction is specified.