Car vortex wedge

I created this a couple of years ago to put on my truck. The intent is to put several accross the back of the cab roof to create a vortex behind. I was hoping to see if this would increase the gas milage by decreasing the vacuum behind the truck. I have divots on the underside to attach neodymium magnets with epoxy so as not to damage the truck in the testing. I never got to use this in the full effect to see if they would work, however the few I did place on the cab held fairly well during my driving.