How fast can you go and still get <acceptable> MPGs?

Last week we were able to take our first ~2000 mile road trip in our young Prius. (Just had it's second oil change this week.) Anyway, not knowing how "broken in" our car really is still at this point, and also wanting to eat up miles to get to our destination as soon as possible while still delivering decent MPGs, I played a lot around with finding the "sweet spot" between max speed and fuel economy. And understanding what others will consider a "sweet spot" to be is relative, here's what we observed:

1) Normally, on road trips, with our other cars, I have operated with 9mph over the limit cruise control setting, except in populated areas which is usually 4 or 5 mph over. (SO I'm one of those people who push the limits of what speed is allowed without making myself likely to be pulled over.)

2) In Idaho, where 75mph is the highest limit permitted on our trip from Utah to the Oregon coast, we tried driving at 84mph. Ambient temps were in the 80s. In some places was in the low-mid 90s. A/C only ever ran as high as the second mark (I dislike the Auto setting so always alternate settings manually). And sometimes, when it got a little chilly we shut it off. While there were some hills and descents, we were traveling on a gradually downward course from about ~4400 elevation in Utah to ~2800 elevation in Boise. (The highest the elevation ever got again was in the Blue Mountains near Pendleton.) Whether on cruise or using the pedal I was not able to eek out higher than a 33 mpg. But dropping cruise to 79 I was able to push out average mpgs to the mid 40s. By working the pedal myself with "pulse and glides" I could squeek out the highest avg mpg of 47mpg. (However, coming back home across Oregon and Idaho, where we had more wind to push against, plus a gradual climb to a higher elevation, keeping the similar speeds I was only able to obtain 43-44 avg mpg.)

SO the sweet spot for our experience seemed to be around 78-79mph was the fastest we could travel and still get what I considered an acceptable avg mpg, which was a goal to at least be in the mid 40s mpg.

Headed across Oregon where the highest we aimed to travel was 9 over 65mph (across eastern part of state--still quite a few hills and mountains) or 5 over 55mph (hilly, twisty areas around the coast) we were able to eek out between 49mpg to as high as 51.5mpg on average. We expected this as slower high speeds most always result in higher MPGs.

But what interested me the most is seeing how fast we could drive, in areas that allowed it, before the Prius' MPGs really came crashing down. (It also makes me wish the MFD would display a graph of average speeds and other useful environmental factors charted against fuel economy.)

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What are others' experiences like?