Daughter needs Gas mileage from 273 commando

You can do a lot of little things to gain mpg like, royal puple oil and gear fluids, electric fan, under drive pulleys, low restrictions exhaust and intake track as longs it maintains velocity, compression, lighting the rotational weight and car weight, lowering the car and 1.5 degree rake and other aerodynamics etc.... but the biggest is to run the car at a lower rpm for any given speed. A 273 running let say 1500 rpms is the same as a 225 /6 running at 1820 rpms but bottom line it takes a certain amount of hp (fuel) to move your cars weight and aerodynamics at a certain speed no matter how much money and technology you throw at it.