I dug out the info for the above cars.
*On the Firebird,the vented hubcap showed a 5.7% drag reduction
*The MOON-esque hubcap measured out at 7.6% reduction.
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With the wind tunnel development of the Subaru XT,the final 'vented' production hubcap showed 0.008 delta-Cd (2.2% drag reduction)
*A completely sealed cover was good for 0.01 delta-Cd (2.75% drag reduction).
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It would probably require a wind tunnel to discern the difference in performance between 'aspirated' and 'sealed' covers.
On-road testing with the slightest change in any parameter,might slew the scatter-plot enough to 'hide' the very data we were looking for.
