The most important thing right at the beginning – all speed limits on signs, in law and everywhere else are given in miles per hour. 1 mile per hour (mph)= 1.609 kilometers per hour (kph).

Speed limits on individual roads vary from state to state. Only on a single highway in Texas you can drive legally at 85mph (137km/h). In the eastern United States the usual maximum speed is 70 mph (113km/h), in the western part of the U.S. it is 75 mph (121 km/h).
https://cheapcarentalsusa.com/the-maximum-speed-limits-in-the-usa

Exceeding the speed limit to 10 miles per hour is usually tolerated on the highway. However, if the speeding is more than that, you can expect a warrant to appear the very next day. The average amount of a fine for speeding in America is 150 dollars.