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You have a point that my math doesn't take into account ground clearance. Air passing under the car doesn't behave the same as air passing around and above, so it's not as simple as just including or not including it.
Another point is that I've simply multiplied the width by the height - the Crown Vic tapers in more aggressively than the Explorer, and is actually smaller than the numbers would suggest.
For the sake of argument, I'll subtract out the difference in area under each vehicle:
Crown Vic - 5.2 inches ground clearance (132mm), 78.2 inches width (1982mm), 225mm wide tires
Ford Explorer - 8.5 inches ground clearance (216mm), 72.1 inches width (1831mm), 235mm wide tires
The Explorer has an area open underneath it of (1831mm wide minus 2x235mm = 1361mm wide, by 216mm high = 0.294mē
The Crown Vic has an area open underneath it of (1982mm wide minus 2x225mm = 1532mm wide, by 132mm high = 0.202mē
The Explorer's corrected frontal area becomes 3.39mē, and the Crown Vic's becomes 2.65mē. Rather than being 30% larger in the wind (not taking into account ground clearance), the Explorer is only 28% larger. Again, not taking into account the Crown Vic's more aggressive inward taper to the roof.
Multiple in the coefficient of drag, and we get a final difference of +42% drag for the Explorer.
~
RPM is significant, as you say, and it's hard to work out through basic math just how much it impacts economy.
Some things I can say for certain:
At 65mph, aerodynamic drag is ballpark 5x larger than rolling resistance. Engine losses are highly variable.
When I put a very large and overpowered engine in my Insight, the fuel economy was around 2x higher than that of the donor car it came from (28mpg highway -> ~56mpg), and this was almost entirely due to it being more aerodynamic.
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