rear wheels - spats or wheel covers?
 

Any research anyone can share on this?

looking to improve the cars aero, either going to go with rear wheel spats (close the arch) or flat wheel covers.

Anyone got any idea which would be better option?

TIA

there is plenty of posts and threads discussing this either on wiki or in forum/aerodynamics. You will find them under the term "wheel skirts" or "fender skirts". It is US forum, after all.
My personal view is this: smooth hubcaps are less effective, but less intrusive and far less noticeable than fender covers. After many curious looks and few pull-ups by local police I dismantled them. In law-bound Europe it is not worth to use them. Otoh, smooth hubcaps are on the safe šíře

Agreed that search is your friend. But if you follow it long enough you'll find Morelli's [European] Punto:

http://i1271.photobucket.com/albums/...titled7_22.jpg
https://ecomodder.com/forum/showthre...ail-33283.html

Here you have an axial fan in the rear wheels that feeds a duct into the spat area behind the wheels. That's the black part in the lower corners, separate from the boxed cavity with perforated annular screen.

The title could have been spats, wheel covers, skirts or air curtains. Your answers should be saying it's vehicle and use case specific. So, that.

I am pretty sure you found this: https://ecomodder.com/wiki/Car_MPG_E...fications_Main

There is written both mods are worth about 3% MPG. As you drive Civic, which is very clean car aerodynamically, there is not much to be done. go for smooth covers, than grille block. both can be made stealth and are not too demanding on fabrication skills.