balls-out vs hell-bent for leather

balls-out

• With great abandon. 

• Descriptive of the use of a centrifugal governor at maximum speed, corresponding to full throttle. 

• At the fastest possible speed. 

• Extreme, extremely great. 

hell-bent for leather

• determinedly recklessly, in a manner that lacks restraint. 

• Synonym of hell-bent (“(recklessly) determined”) 

• Synonym of hell-for-leather (“very fast”)