I dont believe adding wheel covers will make a noticeable difference to fuel economy.
You would get more benefit from removing weight or drag.
At the extreme end of aerodynamics, dragsters and streamliners use smooth flat discs on the wheel rims. But this may have been about stability as musch as drag. On hot rods they're commonly called moon caps. As you have mentioned the exposed wheel and wheel arch has a significant effect. Some attempts were made to address this with wheel spats and covers.
Free things you can do to improve fuel economy are to ensure you have firm tyre pressures and that the car isnt carrying unnecesary weight. (Got golf clubs or bowling balls in the trunk?) And of course your driving technique.
If you want to improve the fuel efficiency of the engine anything that reduces resistance will help. A high flow air filter, headers, an electric fan instead of pully driven, even a simple tune up and ensuring your engine is running properly may help.
Bodywise anything that reduces drag will assist. A body kit may help. Do you need both wing mirrors by law? Are they retractable? You would have to seriously consider the safety risk of removing them or driving without them vs the miniscule saving it would make. But maybe you could get smaller ones.
If you do a lot of flat motorway driving taller tyres will lower rpm for the same given speed, but will increase loads on hills. Conversly if you drive predominantly in stop-go traffic and hills the smaller wheels will lower gearing and may provide more suitable gearing.
However: if this is about being trying to save money i highly reccomend that before you spend anything on trying to improve the fuel economy of the vehicle you should seriously consider the ROI or estimated payback period. If you spend $100 on hubcaps and it saves you 1 cent per mile. You wont be winning until you've driven more than 10,000 miles.
If you look at how much in total you could save in fuel in 1 year with this change. You may be able save more simply by: Not driving as much. Car pool. Ride a bike. Take a train. Buy a scooter. Or maybe consider buying a more economical car.