I have splash guards.. 48 mpg right now, but still working hard on my techniques.

 

the simple answer is "yes, they reduce FE by increasing drag".
the complex answer is more like "to each his/her own".
if you held a splash guard out each window in your hands, the splash guards would create additional drag. so would your arms. And the open windows as well.
Those old enough to recall might remember that the mose fuel efficient Honda's never had a passenger-side mirror. Of course, if you go back far enough (not to floor-based high beam switches, foot-well outside air vents or lap-only seatbelts) nobody had passenger side mirrors - without researching, I think they started showing up in the mid-late 70's, depending on make/model of car. I digress. Back to the question. The Porsche 924 was more aerodynamic than the 944 model - why - because of the flared fenders. The Honda CRX FE was slippier through the air than the normal CRX models -because of little things like omitting the pax side mirror and smooth door window/b-pillars, etc.
I found the responses to this thread interesting because I've found that at least 2 of the 3 local Honda dealers all tack on $199 for a pair of splash guards on every car they sell including the Hybrids. A recent article on a hybrid-themed website [http://www.hybridcars.com/aerodynamic-design.html - I had to go back and find it] illustrates small things that mfgr's do to their cars to reduce the coefficient of drag(the HCH is .28, the Prius II is .26). One of the things they highlighted were the wheels on the new HCH (they reduce drag by reducing the turbulence around the wheel). If you look closely at the sideview angle of a Civic w/ splash guards, you will see that they stick out a bit on the side of the car. They do impact the air flow, in a negative way. That said, they impact is very small. It may change the drag value from .28 to .281.
If you add a front end mask, it will increase the drag. Again, only by a small amount.
My personal problems with splash guards have to do with drilling into the metal around the wheel wells, which opens up a way for rust to develop over time, and it is impossible to clean behind/under a splash guard. That's really a problem if you drive in an area where they put salt down for ice/snow in the winter, esp if you don't clean it out well in the spring. The only way to really keep them clean is to remove them, clean and wax, etc, then replace them. I'd rather clean the original road dirt/tar etc that gets on the body. I had splash guards on my 82 Subaru - you still get dirt/tar on the area behind the tires, just not right behind the guard. It's a waste of $138 (Prius) or $199 (local Honda dealers) in my opinion and just another dealer profit item(like the little stripes and undercoating and wheel locks many add). But as shown by the responses here, some like them, which is fine for them. Thanks for asking the question. I've enjoyed this site, and threads like these, and those that compare a current owner's experience with their Civics to the new HCH or a Prius! YMMV, all IMHO, all the normal disclaimers.

 

I don't believe any drilling is needed to install the splash gaurds.

 

RX8Daniel, it is *possible* that splash guards do not increase drag.
It may in fact direct airflow advantageously to improve drag.

That part of the footpring presented to the air is alredy occupied by the tires.

The air dam is an example of something that is installed in front of the car that increases the area presented to the air, yet it improves MPG.

 

Just to confirm this, no drilling is needed to install the splash guards as I completed the install on my car about a month ago. I was very easy and uses pre-existing holes for screws holding the body together. You just have to remove the old screws and install new ones included with the splash guard. In my opinion great investment for keeping the paint looking new. Without the splash guards installed I believe the lower panels of the car would be damaged from chips/rocks due to the fender shape. I have noticed that the splash guard have picked up a lot of dirt, pebbles and debris in the front lip of the guard (inside the wheel well) where otherwise would hit the paint. They fit the contour of the body well and I put a small coat of wax behind the guards before install. I may once a year remove and re-wax behind the guard for extra protection.