Mud Guards & Bra: Upset Aerodynamics??

Hello to all from a long-time guest/lurker, finally a new member!

First, many thanks to all of you for providing an incredible amount and quality of information. It's been invaluable to me in my pre- and post-purchase phases of Prius research.

Second, a quick intro (I might take time for more later): I'm in central Virginia right outside of Richmond. I bought a new '05 Seaside Pearl, package #2, in 12/05. It has 7000+ miles now. Love the car!

Third, the main point of my post: Is there any way a front end bra and/or, especially, mud guards upset aerodynamics enough to drop fuel mileage signficantly? This past Saturday I installed the black Toyota front end bra and mud guards. The first drive later that day was about a 50 mile round trip across town and back, mainly interstate, 65-70 MPH on average, temp in the upper 60s. I have a pretty good handle now on what fuel mileage to expect (based on MFD display) in varying driving conditions and temperatures, and in these conditions I've been able to get MPG in the low 50s or better. I noticed on the first leg of the trip MPGs (as shown by the MFD 5-minute fuel mileage bars) in the upper 40s. I thought at first I might be running into a headwind. But it was no better on the return trip an hour or so later. Weather was clear and stable (i.e., little likelihood of a dramatic shift in wind speed or direction). In fact, I filled up across town before the return trip and drove more conservatively on the trip, with the MFD showing total of 47 MPG+ for the tankful when I got off the highway.

At this point, my theory is that the mud guards upset high-speed aerodynamics enough to cause the drop. Does this seem plausible or even probable, and has anyone else experienced this with mud guards? And what to some may seem a silly question: Is there any way the bra could have contributed to it? My non-engineer's mind and logic suggest not, but I figured I'd ask. The bra doesn't change the external profile of the car like the mud guards do.

Interestingly, my 8-mile commute this morning, most of which is at 45 or 55 mph, was pretty typical: about 25 MPG for the first 5 minutes as it warmed up and about 50 MPG for the second. I didn't go out at all yesterday (Sunday), so I have no other observations to offer.

Thanks in advance for any responses!

Jim