Lowest RPM's at highway speed?

I am most familiar with the Gen 2, so I will use those numbers.

If you go faster than 42, you have to spin the engine or M/G1 exceeds redline.
You can't get the engine to redine lower than 62 MPH, or M/G1 exceeds redline. (in the other direction)
Once you get to 105, M/G2 is at redline.

Gen 3 added a reduction gear so M/G1 is not as stressed.

 

PRIUS doesn't have a "CVT" like many new cars are coming supplied with (i.e. with a belt drive).

PRIUS has an "eCVT", which has 3 power sources (2 electric motors, one internal combustion engine) driving through an epicyclic gearset, also referred to as a Power Split Device or hybrid transaxle.

I'm not 100% certain that this simulator is perfect, but it gives an idea - figures for Gen 4 will be different:
Toyota Prius - Power Split Device

The WIKI article is very informative:
Hybrid Synergy Drive - Wikipedia

If you have a bit of time to spare (1½hrs), John Kelly has a full breakdown and explanation of the Gen 4 Hybrid transaxle:

[If you want to see a "conventional" CVT operation, search for "weber cvt" in YouTube - he's got a few different videos showing how they work.]

 

The Prius' electric CVT allows ratios all the way from zero to infinity, much wider than common mechanical CVTs. This is why you cannot meaningfully pin down Prius engine speed based upon wheel speed.

Older Prius generations had RPM limits on certain components that required the ICE to spin at highway speeds for mechanical overspeed protection. Gen4 pushed those component limits higher, to where the ICE is no longer required to spin at all at (lower) highway speeds.

 

Welcome to Prius Chat. You're just learning that your new Prius is far more complex and refined than you ever imagined. Enjoy your new car and the new learning curve it's presented you with.

 

73 MPH is the fastest she'll go without ICE.

 

Is that the non-Prime version? I've heard a higher speed as specific to the Prime.

 

That's the speed above which the ICE must come on, no matter how favourable the conditions for just EV propulsion. Oh, and that's just the Gen 4 hatchback!