https://calteches.library.caltech.ed.../MacCready.pdfIf this links, it will get you to an article about the AeroVironment/ GM Sunraycer development.
On page-5, you'll see an incomplete image of the 1/4-scale wind tunnel model they used at the GALCIT tunnel at Cal Tech University.
As of 1987, this model registered the lowest drag of any land vehicle ever measured, @ Cd 0.089.
In the photograph, you can just make out the full wheel fairing at the front wheel, part of a complete package.
Concerns over crosswind gust stability of the 573- pound ( 260.4-kg ) total weight car led to removal of the entire wheel fairing package, which degraded the drag to Cd 0.125 @ zero-yaw.
The car was later yaw-tested for SAE crosswind-averaged drag, with Cd 0.147.
--------------------------------------------------------------------------------------
The wheel fairings reduced drag by 36-counts ( 28.8% ), an approximate 80% drag reduction for the exposed wheels.
In Goro Tamai's book, 'The Leading Edge,' he suggests that full, swept fairings could account for up to a 70% drag reduction of the wheels.
Sunraycers' wheels are more exposed than many solar cars that would follow after 1987.
-----------------------------------------------------------------------------------
Dr. Paul MacCready, who ran the program, estimated that a modestly-powered, internal combustion variant of Sunraycer would return 400- miles per gallon, highway.
At 90- km/h, Sunraycer's total road-load power requirement was 2,000-Watts ( 35.76- Wh/Mile ), at Cd 0.125, 14.222 square feet frontal area ( 1.321 meters-squared ), and Coefficient of rolling-resistance of Cfrr- 0.00653.
-------------------------------------------------------------------------------------