If rocket engines can reach 70% efficiency, why don't we use them in cars?
Internal combustion engines get less and less efficient with higher speed just due to the kinetic energy equation (a unit of fuel produces a unit of velocity^2), while rocket engines have a constant efficiency (a unit of fuel produces a unit of velocity), ignoring fuel consumption.
This is a fundamental constaint, since an internal combustion engine necessarily pushes against the moving earth to propel the car, and a rocket engine effectively pushes against a gas that is stationary wrt the rocket.
So why not use a rocket engine in a car? Wouldn't it be more fuel-efficent?
reference for 70% figure: https://en.wikipedia.org/wiki/Propulsive_efficiency