That depends on which state you had in mind. In Colorado, it's 70 for most of it, though it slows down to 65 through Colorado Springs, and down to 55 in some parts as you're going through the Denver Metropolitan area.
On rural areas it varies from 70 to 75 MPH.
On urban areas it varies from 55 to 65 MPH.
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.
60
65
70
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.
The average speed limit in the United States is typically around 55-65 mph on highways and interstates, and 25-35 mph on urban roads. However, speed limits can vary greatly depending on the specific location and road conditions.
70
70 MPH on Interstate Highways.