[aerohead]

Here's some past published data, just to muddy the waters:
1) for the Cd 0.312, 1982 Pontiac Trans Am Firebird,
* the OEM ventilated convex covers, compared to the naked, welded pressed- steel wheels, reduced drag by delta-Cd 0.021.
* by taping all the openings closed, delta-Cd 0.024.
* with John Shinella's top-secret, convex wheel covers, delta-Cd 0.027.
--------------------------------------------------------------------------------------
2) for the Cd 0.29, 1985 Subaru XT coupe:
* compared to the naked pressed-steel wheels, the ventilated, flat disc covers reduced drag by delta-Cd 0.006.
* by taping all the openings closed, delta-Cd 0.008.
--------------------------------------------------------------------------------------
3) The 2012 Jaguar XJ Supercharged was wind tunnel tested at MIRA, by Coventry University:
* when the OEM, 10-spoke, alloy wheels were covered 100% with flat discs, drag was reduced by 3.3% ( Cd 0.29 - Cd 0.2804 ).
------------------------------------------------------------------------------------
4) From the 2018, 8th International Conference on Advanced Concepts in Mechanical Engineering, Case-4, a CFD-generated, generic hatchback, of Cd 0.377 was aero-modified with full rear skirts, plus 100% coverage flat wheel covers, covering the 'OEM', 6-spoke wheels, resulting in a delta- Cd 0.024 drag reduction ( 6.36% ) for the two modifications.
* Coventry University had tested the U.S.- spec, Cd 0.288, Audi A2 @ MIRA, with full rear skirts, and measured a delta-Cd 0.038 drag reduction , over 8-spoke wheels ( a 13.19% drag reduction ).
------------------------------------------------------------------------------------
5) The 1985 Pontiac Trans Am Firebird, with aluminum turbo wheels was Cd 0.32.
With the standard aero wheel covers, Cd 0.29.
-----------------------------------------------------------------------------------
6) In 2012, Coventry University measured an Cd 0.316, Audi A3 @ MIRA, with full rear wheel skirts, and realized a delta-Cd 0.006 drag reduction ( 1.898% ).
--------------------------------------------------------------------------------------
7) The 'spread' for rear wheel skirt delta-Cds between the Audi A2, and 4)'s generic hatch is so great that, it makes parsing out, and isolating, the drag reduction contribution for the wheel covers, with any confidence, too difficult.
--------------------------------------------------------------------------------------
Perhaps a member, or guest, which has access to an 'aero- wheel covered' Model 3 could perform a 'taped' comparison, and report.