It can be from 60 to 85 mph.
In most places in the US you are not allowed to exceed the speed limit by any amount at all when passing. It's called the speed LIMIT, not the speed SUGGESTION. If you can't pass without exceeding the speed limit, then don't pass.Some states used to allow this, at least under certain conditions, but several of them have since changed their laws to forbid it. If you want a definitive answer, contact your state government; the highway patrol should be able to tell you for sure.
the speed limit is 65 mph, 70 when it is posted
The maximum legal speed limit on a rural Interstate highway is the posted speed limit in the United States. Each state in the United States has different posted speed limits for Interstate highways and other highways.
Any type of measurement is a unit rate. The speed limit is a measurement of time and distance.
around 65m.p.h on the straightaways
The highest speed limit I know of on an Interstate highway in the U.S. is 75 MPH, but that varies.
A scalar, since no direction is specified.
70mph on some rural interstate highways
70 mph - but some states do not allow that speed.