Racing bicycles have flat disc wheels instead of the open spoked type found in everyday bicycles. The reason is because flat disks have less air resistance. And as speed increases, air resistance quickly increases until it becomes the limiting factor.
The same principle applies to your car. Smooth wheel covers will help streamline the airflow around the wheel well area, lowering the air resistance. Less air resistance means you will get better fuel economy. The next video demonstrates a simple homemade wheel cover. It doesn't have to be fancy. From the point of aerodynamics it only has to be a flat surface. You can even use pie plates!
You can also try to earn a little advertising money with your wheel covers. There are companies like CapAds which sell smooth wheel covers that carry advertising. The wheel cover is designed to stay still and not rotate so the ad can be clearly seen. From the point of view of aerodynamics the important point is that the flat advertising space acts to reduce air resistance! The following video shows a car equipped with advertising wheel covers.
There is a saying that there are no such things as magic silver bullets but there are silver BBs. The idea is that in the real world you can't just push a button and solve big problems like reducing your fuel consumption. But often there are a lot of little things you can do that add up to a solution. The silver BB solution requires work because each individual BB requires a little effort to implement. Just as the benefit of the BBs adds up to a solution, the work required adds up too and can be in total quite an investment. But at the end you have the problem solved. So if saving on gas is a problem for you, consider adding aerodynamic wheel covers to your list of BBs.