[ECONORAM]

I have tried a few flavors of material. Clear packing tape (doesn't stay long at 32*), cardboard, corroplast, and LEXAN. The corroplast worked fine, but you can't see the wheels... 0.100" LEXAN cost me $90 for a 4x8 sheet, and I got 8 covers out of it. The RAM's wheels are 20inch, so 8's all I could squeeze out. I've also tried 0.020" LEXAN, but it is not strong enough to withstand the aero forces present at the front wheels. I lost one last week while driving 70 into a strong headwind. 0.100" is a bit stiff, as it it hard to get the straps around the wheel spokes and fed through the covers. I might try some .050" if it bothers me too much.
One thing to remember. I also learned to stick some foam window insulation strips to the back side of the covers where they contact the spokes...to keep from tearing up the surfaces.
[edit] I did an informal test about 18 months ago on I-44. Had a crosswind and thought the covers were giving me a vibration problem. So I stopped and removed them. Once rolling again, I found I had to slow down 5 mph to maintain the same fuel economy. I was getting 16.5mpg at 70 mph with covers; 16.5mpg at 65mph without. Rough math says that's about 7%.