Interestingly enough, when I drive 70mph vs. 65mph on the freeway, I get better gas mileage. Most of my freeway driving is mostly flat with a few not-so-steep hills. I don't use cruise control as I did in the past. I think the additional speed is giving the vehicle more momentum so when I come to hill, I can decelerate while still keeping up with traffic and getting about 60MPG going up the hill believe it or not.
I expect that the best speed for high FE is below 60mph due to wind drag building to high levels above that speed.
It is for the HCH2 and probably is for most vehicles.
Traveling below 60 mph on interstates here would be very difficult to maintain without becoming a hood ornament. It would be nice if they would do a better job at enforcing the posted limits. Maybe they should bring the limit back to 55 to slow folks back down to 70 instead of 100+
It could be that at 70 MPH, you're less concerned about maintaining car speed - i.e. when the road turns uphill, you can maintain throttle position (and fuel consumption) and allow speed to drop reasonably until you reach the crest. At lower speeds, you might be more concerned at maintaining speed, for fear of becoming a road hazard, and consequently you increase pressure on the throttle to compensate, increasing fuel consumption.
I've noticed in my Passat that I can maintain higher MPGs at 42 mph than, say, at 37 mph, because I can only force it into fifth gear (Tiptronic) at speeds greater than 40. Sometimes I find myself accelerating more quickly just to reach 40 mph/5th gear.
If you're comparing steady-state 70 miles per hour to steady-state lower speeds, you're bound to get worse mileage, all things otherwise equal. (See 1stpik's post.) The Prius MPG Simulator verifies this. (It calculates the effect of speed and many other variables on MPG.)
On the other hand, your post suggested something other than steady-state. You may be (somewhat intuitively) keeping the vehicle's RPM in efficient operating ranges by allowing speed to increase on the downhills and to bleed off on uphills. Some advocate a specific technique called warp stealth on hilly highways to improve mileage at highway speeds. I have tested this on an interstate highway with gentle hills, comparing WS at an average speed of 60 MPH to allowing cruise control-maintained 60. (Results not posted .... One of these days I'll finally get the last edits completed!) With those identical average speeds, the results were nearly identical. WS might still be of some benefit on terrain with steeper hills. Hobbit (the author of the linked WS paper) certainly believes it.
I notice you just recently bought your Prius, so presumably your highway driving in it is still rather limited. You can't judge your results on such a limited experience. Many other factors affect fuel economy, including weather. Temperature and winds, in particular, can have a significant impact. A cross or head wind as little as 5 MPH, for example, can drop it by 8%.
Lifetime fuel mileage: After learning how to hypermile: