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I think this would depend a lot on what type of car you are talking about.
A lot of Land Speed racing cars use inside wheel covers... but only the Lakesters (cars that have exposed wheels outside of the body). With a typical passenger car, only a small percentage of the wheel is below the level of the air dam (if any at all) so I would guess the gains to be so little as to be unmeasurable. For a truck that sits up a little higher and the wheels are a little more exposed, maybe. But then, the first thing you should do for mileage is lower it and put on an air dam...
My vote is: don't bother.
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