It would be hard to quantify. I'd design your cooling system your way, but not trunctuate your aerodynamic efforts too much in anticipation of a huge gain.
In other words, hedge your bets. Make your wake small to begin with, and then see how it fills with your blown air. You cannot deny TANSTAAFL
You can do tuft testing which I think is your only recourse to see how it works.
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2007 Dodge Ram 3500 SRW 4x4 with 6MT
2003 TDI Beetle
2002 TDI Beetle
currently parked - 1996 Dodge 2500 Cummins Turbodiesel
Custom cab, auto, 3.55 gears