I have the secret right here:
Suppose you want to do 300 mpg, in a vehicle that goes 60 mph. Then a gallon's journey, 300 miles, takes 5 hours. A gallon of gas contains 115000 btu of energy, or about 121 megajoules. This energy is burned in 5 hours, for a rate of 24 MJ/hr, or about .00667 MJ/sec = 6.6 kJ/s =6.6 kW. One horsepower is about 750 Watts, so a 100% efficient 6.6 kW engine would be about 9 hp - about the actual power delivered by a small riding lawnmower.
Now lets consider a man on a bicycle. 10 mph is not too hard to sustain for a rider, about 200 W output. Air resistance goes as the cube of velocity, so going 60 mph would require 63, 216, times the power. 200 x 216 = 43200 W or 43.2 kW. Our hypthetical vehicle needing only 6.6 kW do 60 would have to be about a sixth as hard to push as a man on a bike. Have you ever pushed your car?
The "secret" to such a vehicle is that it would have to have less air resistance than a man on a bike, and not much heavier. Such a vehicle would be extremely light, fragile, and especially unsafe at highway speeds, and would be blown off the road in a good crosswind.
This is the secret that the Big Three have locked up in their safes ... that a vehicle with a lawnmower engine weighing less than a moped and with much less air resistance than a man on a bike, will get 300 mpg.