@40 MPG at 67??

If you drove 53mph or less here in California, you'd either get shot at or rear ended by someone driving "normally" at 80mph

 

Is it possible that perhaps your air guage is faulty and reads high. Make sure you have at least 38 PSI. I find that 38 PSI allows for easily obtaining 50 MPG + tanks regardless of situation (speeds 70 MPH or below)

 

On a recent trip back from Myrtle Beach on I-95 (2500 miles on the ODO), 65-80mph, on/off for rest stops and lunch, some stop and go around the DC loop, air conditioning on, STOCK tire pressures, pretty agressive driving, and I got 49.8 over 475 miles calculated at the pump with simple math and a pen ..... not the trip computer. Make sure you set your tire pressures COLD and use a good guage.

 

I've been driving on California freeways at 55 mph for the last 2 years and haven't been shot or rear ended, both during commute times and on weekends.

 

If you disconnect the battery, you will clear the parameters that the car uses to adjust the MFD mpg calculations and also the fuel trims used to adjust the fuel injection.

My experience is that when I disconnected my battery (I did it twice when working on the drivers seat) my mpg went down both as reported on the MFD and by how far I drove until I needed to refill again. After 2 or 3 tankfuls the mpg returned to the previous mpg.. YMMV.

 

(YMMV) Your Mileage May Vary.

 

I averaged 77 mph on a 800 mile trip, meaning I spent a lot of time at speeds above 90 mph.. I actually hit 105 mph a few times.
My MPG was 39.8.

Never once did I feel I wasn't in control of the car.. I did feel slight cross wind but overall I was impressed with smooth ride, considering skate board size tires.

 

Tire pressure is whatever the door sticker says.

Funny thing about the trip, I normally see 10% of the traffic at speeds at or above 80 mph.. This trip 99.999% traffic was below 75mph. The only 2 prius I came across were both doing 85 - 90 mph, one going and the other one on the return leg.