Aerodynamics & Diverting Wind Resistance ?

Wind "through" or Wind "around" ??? How could/should a guy go for a gain ?? I have a couple ideas, seeing what else is out there......
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And at what "speed" or MPH before it matters ??
And even then, how MUCH does it matter ???
And, is there a small change to a '80 D150 front end that would be beneficial - behind the grill ??? .... keep'n it stock appearing
Read up on the Tatra 77, most aerodynamic car of its time