Why racing cars have a streamline design?

Stream·line

[ˈstrēmˌlīn]



VERB

• (streamlined)
  
  design or provide with a form that presents very little resistance to a flow of air or water, increasing speed and ease of movement:
  
  

Duh!

Think of air resistance, yes, air does have mass. Look at a Formula 1 racing car and then a moving truck. On Earth it makes a big difference, in a vacuum, deep space for example, it makes no difference.

Ever watch international barn door racing?