I'm just curious if anyone has the Prius mud flaps/splash guards installed, and if so, how has it affected your gas mileage?

no difference. I have a Canadian spec os mudguards are standard and living in BC, I can get 4.0L/100km (59mpg) 100% city driving of course.

Mods: EV mod, VVT-i emblem, sport pedals, OEM cargo mat, JDM Prius interior footwell lighting, DICE iPod kit, OEM all-weather mats, LED licence plate lights, Phillips Vision Plus

Unless you do triple digit speeds, the splash guards give so small a hit on fuel mileage that you will never notice it.

Has anyone done any detailed studies on this?

if you don't have the mudflaps you likely have to wash your car more often

washing your car uses energy

water consumption is energy

(and don't forget the crap you are washing into your local creek, if you are washing at home) (but don't get me started on the environmental advantages of commercial car washes versus the home approach - still, I wash at home around 50% of the time)

No, but think about how little the profile is for them. The aerodynamic portion certainly has less effect than say the side mirrors.

True. So the next question is, does anyone drive with their mirrors folded back?

food for thought .. see also
Originally Posted by wjl Splash Guards/aka: mud flaps

I would like to see the mirrors much smaller! I have thought of turning them in, but I don't think we would notice a differences? I think a good wax job may help! H

Mudflaps will have no effect on MPG, BUT they will help protect your car from small rocks, etc. My previous car was a Honda Fit Sport and I didn't have mudflaps...I had small dings along the bottom side of the car due to rocks and other debris.
I had flaps installed on my Prius, ding and dent free! I also had side moldings put on my doors because people don't care if they hit your car door or not!