Gas Mileage

It really depends on your topgraphy, climate and driving style. I just got my first 50mpg tank (450+ miles) last week after
a) cool temps
B) engine break-in period
c) learning how to drive the Prius efficiently to maximise its potential

"Feathering" is a good technique. Remember, the computer is only as smart as the person who programmed it and the person who uses it.

 

Mid-40s are not realistic at those speeds under most situations. Let me give you some example mpgs that you could expect:

The following are the baseline conditions for these calculations:
85% humidity, no wind, elevation 1005 ft., AC on set to 72, summer fuel, 600lb load, OEM tires.

@50 degrees:
75mph: 42mpg
85mph: 37mpg

@70 degrees:
75mph: 46mpg
85mph: 40mpg

@90 degrees:
75mph: 48mpg
85mph: 42mpg

Now, add a little wind (10mph head wind @45 degree angle) and things get ugly:
@50 degrees:
75mph: 35mpg
85mph: 30mpg

@70 degrees:
75mph: 37mpg
85mph: 32mpg

@90 degrees:
75mph: 40mpg
85mph: 34mpg

Those numbers are all calculated using a program created by Wayne Brown. And, they should be accurate within a couple mpg depending upon the rest of your commute (city driving distance, warm-up time, etc.). But since you say it's 42 miles and almost all interstate those numbers should be pretty realistic for you. 85mph is the killer here. If you could bring your speed down to 65 or 70mph it would really help the mileage.

BTW, the Prius, and other hybrids, will do as the EPA number suggest, but when you see drop the percentage change may be the same as with a non-hybrid, but you notice it more in the absolute numbers so it seems more dramatic.

For a pure commuter vehicle of the distance you drive the Insight might be worth your consideration...if you don't need seating or cargo space.