I’d just like to point out that at 70 mph, the hybrid drivetrain in a Prius is nothing but a heavy anchor. It’s not doing you any good as far as mileage is concerned.
At that speed, the gas engine is tasked with 100% of the effort of moving a 1.5+ ton steel box against the resistance of the air, the road, gravity (real world roads aren’t completely flat), and everything else. For an I-4, 1.8 liter, 121 hp gas engine to get 50 mpg under those conditions seems … overly optimistic, to put it mildly.
That’s just simply physics, as I’m sure you know. Toyota even tacitly acknowledges this, if you look carefully at the published data for the car (quoting from the 2017 Prius specs ):
EPA Fuel Economy Est - Hwy : 50 mpg
EPA Fuel Economy Est - City : 54 mpg
Fuel Economy Est-Combined : 52 mpg
What’s wrong with this picture? It’s the opposite of what we’re used to see, for a car solely powered by an internal combustion engine.
That’s because in city driving the hybrid drive train can really help. It lets you reclaim some of the energy spent by the gas engine accelerating the car between lights (for example), by using regenerative braking. Not so on the freeway at 70 mph. You are constantly pushing against resistance far outstripping the output of the electric drivetrain and hardly braking at all.
You see the same phenomenon reflected in the published range of the car:
Cruising Range - City : 610.2 miles
Cruising Range - Hwy : 565 miles
The Prius shines in the city, not so much on the highway. In addition, as Sandy pointed out, the EPA highway stats are measured at lower speeds, nowhere near the 70 mph we’re discussing here. To quote from Wikipedia :
Under the assumption that the fluid is not moving relative to the currently used reference system, the power required to overcome the aerodynamic drag is given by:
Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome aerodynamic drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting 4 times the force over a fixed distance produces 4 times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, 4 times the work done in half the time requires 8 times the power.
(“Fluid” here being used to describe air. Emphasis mine.)
A large gas engine on the other hand, tuned to a high level of efficiency (by gas engine standards, anyway), turning at low revs and encased in a light weight sports car body with relatively low drag, can get excellent mileage under the same circumstances. That is, provided my lead right foot is kept out of the equation. I.e., you have cruise control engaged, a lock-up converter, fully electronic engine and transmission management, the works.
Would the sports car get better mileage than the Prius, even at 70 mph? That depends on multiple factors, such as aerodynamic drag of the two cars, head or tail wind in each measurement, the type of tires used, how those tires are inflated, weight of the vehicle, engine tune, and the driver’s driving style. But no, it’s not likely, even though it might be a closer race than you’d think.
Please also keep in mind that all the stats mentioned in this thread are anecdotal, with the exception of the published numbers from Toyota. “My friends says he gets 55 mpg at 70 mph” is the epitome of anecdotal data. Not only do we not know the exact circumstances of said measurement(s), but there’s also a lot of opportunity for personal bias to creep into the equation. If you bought a Prius to save gas, you’ll want to defend that investment by extolling its gas-saving virtues at every opportunity, including sometimes even fooling yourself. It’s just human nature.
 Toyota’s 2017 Prius specs:
 Wikipedia article on drag and power: