+1 Those differences could be accounted for by changes in topography and wind direction/strength. Multiple runs over the same route, like daily commuting, over a tank of gas or more for each condition -- being careful of long term temp trends over the period of observation and large speed variations due traffic, etc -- would tend to nullify such variations. A very difficult trick to pull off with reliability/repeatability... And there's the 1 PSI variation of tire pressure due 10 degF change in air temps... Variations could also be anticipated with tires with less than 3-5K miles as they break-in. For now, I judge the possible aero contributions of the wheel covers to still be unquantified.
If it really is about aerodynamics, why wouldn't Toyota just design an aerodynamic alloy wheel (like the civic hybrid)?
Then it would look like the pizza/pie wheel on the Civic Hybrid. Toyota's trying to blend a little design and aerodynamic... trying not to make it look like a disc (which is what Honda did with the HCH-II) or to make it look like the 17" alloys. Note Honda did the same with the Insight-II. The NA Insight has more aerodynamic alloys than the European model but even then, it looks pretty nice compared to ours (with the wheel covers on) Here's one from the Official Pictures thread
Check this thread out http://priuschat.com/forums/gen-iii...ons/79133-3-buck-alloy-wheel-center-caps.html. I can vouch for the 1 1/2" knockout seal solution. You just need to bend the prongs out a bit, and voila! I got a pack of 4 at Orchard Supply Hardware for $4.60 or something like that. They seem better than the ones I saw at Home Depot - they have the size stamped on them.
