If you are getting 19.2 mpg at any speed in the Versa I would say something is wrong. I have never been happy with the fuel economy in my Versa (2007 with 4 sp auto), and have consistently averaged in the mid 20's and get in the upper 20's (mpg) at highway speeds. In my other four vehicles I can always hit the highway fuel economy number or above even in city driving.
Some good advice has already been offered. Check your math and base your conclusion on several fillups to factor out differences from filling up. Slowing down a little from 77 mph (125 kph) should make a difference as the wind resistance increases as a square of the speed. However, I have found my Versa loves the 3000 rpm range on the highway.
Some bad advice: Raise the air pressure in your tires 20 -25 % above what is recommended by Nissan. Not only will the ride in the Versa go from bad to terrible, but the risk of blowouts rises. Tires will wear badly, handling will suffer, wet weather traction and winter weather traction will become scary dangerous, and the suspension is taking a pounding. I know others have said this is good, but seriously, don't follow their advice. Raising the air pressure from the recommended 33 psi ( I think) to 40 will not raise your gas mileage from 19 to 30. It may go from 19.2 to 19.5 but the reduction in life to the tires and suspension components will far exceed the value of what little gas you save. Besides the Versa is already a rattle trap, I can't imagine shaking everything even more from a 40 psi ride.