Welcome, Warren!
I have several "mates" in the land down under.
Herb Hanna, Peter White, etc., so I hope you get to fly with them, if you haven't already.
I totally agree with the guys who have already posted. They do reduce the drag of just having a wheel/tire hanging out in the breeze, but they mostly seem to serve an "beauty purpose".
a couple things to be cautious of: make them as light as possible, fuel proof the insides, and MAKE VERY SURE that they are properly aligned. If they get skewed in or out, it will cause trim problems that you may never catch.
Oh, you can make them take apart so that you can change tires when you wear the first set out!
Bill <><