Welcome to Prius Chat and congratulations on your new purchase. The hub caps are for aerodynamics and mpg. A lot of us on here take off the outer plastic covers and add a cap to the center hole. I have not seen any appreciable impact to mpg. Toyota went to great lengths to lower coefficient of drag and increase mpg.
Yep. It was for aerodynamic purposes (the idea was that the air flowing past the wheel is smoother and doesn't interfere with the air coming out of the wheel well). It could also be for weight savings since plastic is still lighter than alloy (even though the wheels are lightweight alloys). Some of us have taken it off because we like the look of the twin-spoke alloys underneath, and like JD said above, added a centre cap to complete the look.
Some people are buying covers from Home Depot. http://priuschat.com/forums/gen-iii...ons/79133-3-buck-alloy-wheel-center-caps.html
You can get a set of 4 from Ebay for about $10, shipping included from China, Chrome plated plastic caps with the Toyota Logo on them!
That's the first thing I've noticed too and there are large debates on here about it. I just think its pretty neat so I'm keeping them hybrid hubcap/rim thingamagigies.
On the 2nd gen, the plastic trim rings seemed to serve another purpose. When you have a flat and use the tie down straps, they seem to prevent the alloy wheels from getting scratched by the straps.
I got OEMs so I got them from the dealer (not cheap!). You can get them off ebay or warehouses that sell Toyota parts.
