My understanding is the Prius or any other hybrid gets lousy gas mileage ... running on pure gas alone. It’s the combination of the fuel/electric that gives it its economy ....
The hybrids are also able to boost higher than realistic EPA mileages because of the way the EPA tests them. They institute a max speed of 55 and accelration of 3.3 mph per second.
No one drives like that. Give it a try some time. Best you can't do it.
I think the advantage of a hybrid is in city driving, where the electric motor is more efficient than the four-stroke. Highway, it’s less than a wash, slightly worse.