The only really dumb question is the one you don't ask. Yeah, if you put OEM mud guards on your Prius, it'll cost you a little in the MPG department. If you're a hypermiler, you might even be able to measure the difference. I'm presuming that you mean OEM units, and not some kind of Redneck inspired ground-draggers, but yeah.....if you're really in need of mud guards, then aerodynamics is the very least of your MPG concerns. If you still have the plasticky wheel covers on your OEM wheels, I'd consider removing them and putting a set of $20 metal covers in the center holes (or $2 1.5 inch electrical box knockout covers from HD.) Yeah...it'll cost you another 1.something MPG on the highway, BUT...if you're playing around in the mud that much...you'll want to have the ability to clean out your wheels (and brakes.) Just a thought. GOOD LUCK!!!
Get the smaller mudguards at the Pshop if you want hardly any mpg hit, they don't stick out further than the tires and are quite small. But the mud deflection is somewhat minimal compared to the Euro larger versions, though I do see a significant lessening of streaking on my white Pri after installing the small ones.
OK, I'm going to take prius on about 20 mile stretch of interstate by my house at 65mph, and I'll trip miles at a certain place once I get up to speed. I then will install mudguards and make the same trip, trip at same place and see what the difference look like in MPG. Now I know a tail wind would make a difference but can't control that.
The smaller ones on this web site are the one I was looking at. The euro are fit higher up on the panels and I hear they are harder to install also.
