You made it less aerodynamic.
Bigger front-dam profile.
Bigger "suction" area in the back end.
I'm not a pro with the graphics programs, but you need to have a gradual drop in the back, instead of the big flat end.

That's the same reason a pickup truck with a cover bets better mileage than one with an open bed, and an open bed gets better mileage than a pickup with the tailgate down.

The gradual step down is more aerodynamic than the big flat edge of the rear of a pickup cab.

(Waiting for the doubters)

[color:"white"]? 04 Rodeo DI ?[/color] 75k mi, body damage on the 1st weekend I got it.