These are the smooth/racing hub caps. Some call them Bonneville salt flats cap. These caps are stainless steel, but are available in aluminium too.
I've heard reports of better MPGs with this style cap. I'll believe it if it works.
In the video I showed to cut about every 5-6 of the "fingers" , well don't do that right off the bat. You want to cut a few as possible until the cap will go on the wheel. That will keep the cap tighter on the wheel.
Im going to get a long spring and loop it through the wheel then hook it to the cap to insure the cap stays on the wheel. I'll make another video of that later.
www.leroydiesel.com