I've driven the I-15 from San Diego, CA to Pocatello, ID about 20 times over the last year. The top posted speed limit is 75 MPH from North Las Vegas to exit 260 south of Provo, UT. The only slowdown is the segment from Littlefield, AZ to St. George, UT where the winding Virgin River Gorge makes slowing down necessary for safety. In all those trips, I've seen few drivers anxious to exceed the 75 MPH posted limit. There is a construction area immediately north of Beaver, UT where the speed is limited to 60 MPH for a 3 mile stretch because of construction equipment. Be advised that 18 wheel trucks maintain a 75-80 MPH speed in Utah and Nevada. There is no lower speed limit assigned to big trucks in those states as one finds in California and Idaho.
The San Diego area has max posted speeds of 65 MPH on I-5 and I-805. It is rare to see people drive under 70 MPH when the traffic flow will allow it. CHP simply doesn't have the manpower to ticket everyone violating the law. If you actually try to OBEY the speed limit, you may get a citation for dangerously impeding traffic flow.