I also think you're correct in suspecting that removing the air dam and not installing an undertray will hurt FE.
But I also suspect that even with a good undertray, eliminating some air from under there by using a good air dam might have it's own benefits.
For any given space volume under the car, more air banging around under there will give more drag and less air should give less drag. At least that's my best guess.
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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