Hi,
Let's start with some data:
This chart includes some high-speed driving data from an NHW20 model (your Prius) last summer. As you can see, speeds of 70 miles per hour and higher are pretty rough on both the NHW11 and NHW20 Prius mileage.
Many states have a 65 miles per hour trucker speed limit and some trucking companies have policies of using 65 miles per hour. If you can find one of these trucks, follow them at a comfortable following distance and set the cruise control. You'll get great mileage and still have plenty of time to respond to road debre. Don't follow gravel or construction equipment haulers because they shed bouncing gravel and rocks.
Looking at lower speeds:
There is a boundary speed, 42 miles per hour, between low-speed ICE/EV mode and ICE-on always mode. If you can stay on one side or the other of 42 miles per hour, you'll avoid unnecessary ICE start/stop events. In city driving, try to avoid using the brakes either by route planning or predictive braking. Predictive braking includes a light braking, an early touch, when approaching a red light so you maintain some momentum for a little bit longer. You're giving the light a chance to change so you don't have to come to a complete stop.
One last trick is recognize the engine becomes less efficient at higher rotation speeds. If you hear the engine rev up, try to reduce the accelerator, IF POSSIBLE! One trick when approaching an overpass is to speed the Prius up just a tad and then let the speed decay, not rev up the engine, going up the overpass slope. Then just
after the crest on the backside, pick up the speed again. It takes a little patience to 'wait for it' but it pays off in fuel savings.
As a general rule of thumb, try to increase speed on down-grades when gravity helps. When going up a slope, keep the speed constant or better yet, let a little bleed off.
I hope this helps,
Bob Wilson