While it is certainly true that internal combustion engines tend to operate more efficiently when the fuel is well vaporized, there are limits to how much mileage benefit there is. Think about it- your fuel injected Neon doesn't get 78 mpg, does it? So why would you think it's possible to get 78 MPG out of a 400 inch v8, no matter how perfectly the fuel is vaporized?
Or to look at it more scientifically, vaporization only helps ensure
100% combustion and prevents/reduces the amount of unburned fuel that gets sent out the exhaust pipe. But even with PERFECT vaporization, you still need to burn enough fuel to overcome friction and air resistance, and allow for the other loss mechanisms (such as thermal loss from the combustion chamber to the cylinder walls). Some benefit can be had from leaning the mixture beyond stoichiometric (the perfect amount of oxygen to burn each and every fuel molecule), but not much and you begin to trigger excessive NOx emissions.No, further improvments in fuel efficiency aren't going to come from miracle carburetors or fuel injectors. They're going to come from more optimized utilization of the engine's power curve (hybrid cars, for example, once we get better battery packs) and better thermal efficiency inside the engine (insulating coatings inside the combustion chamber, higher temperature engine materials that can survive without cooling, altered combustion cycles, etc.)