I have been doing testing on these flaps a lot lately and they do effect the consumption a lot.
I took 10mm off from each flap after this video and consumption got back to 0,1liter higher to 5,5 liters.
Flap width is usually narrower than the tire. Do notice that inner side is wide because it also directs air away from the suspension lower a arms etc. just 10mm changes to shape and size can have measurable impacts.
Tip is that build adjustable flaps in heigth first and get the heigth best setup After that build flaps that you can adjust the width also.
Optimum heigth of the sparts are aroung 40-70mm. And width needs to be about 40mm.
Conserning the A arms the best heigth is around the lowest point af the nearest body structures behind the flaps. Usually lowest point of the body.
You have to be able to test these mods in calm conditions as wind do effect fuel consumption of the results close to 10% and easily 5%. So best is that you try to find level road for 3-5km and test the consumption to both directions. make fast changes and repeat the test. Try to keep measuring speed high so its easier to see the differences. I test usually at 100 km/h, but I will try the method I described and will test at 120 km/h to see better results. To get the smaller differences. I also keep records of the consumption at various points on my test route to see if the consumption changes earlier or later compared to previous setup. That is because in Finland near me is not perfecly level roads so there is up and downhills on the route.
I will also try to test to put angled flap when I have found the best setup if that brings even more gains.