You know what I'm referring to, those plastic deflectors mounted on the tops of our vehicles' side windows.

In other markets, they call them rain guards, vent visors, or window deflectors. Here, I think we typically call them rain gutters.

We all know that increasing wind resistance and drag can significantly increase fuel consumption, especially when traveling at highway speeds.

My question is, do rain gutters increase drag?

I tried researching this but I came up with conflicting results. It seems no one has bothered to measure the aerodynamic effects of rain gutters.

http://www.car-stuff.com/acuraventvisor.htm

Vent visors are basically considered as a type of spoiler, working to improve the aerodynamics of the automobile for higher acceleration and easily reaching top speed. Their shape reduces wind drag, aiding then to achieving better fuel mileage.

http://www.expeditenow.com/news/publ...s_AirTabs.html

To reduce air resistance and drag, the first thing you need to do is eliminate anything you can which causes resistance in the first place. Side mirrors and the brackets cause quite a bit of resistance, but you don’t want to remove those. An exterior sun visor is a major factor in wind resistance, and many people have removed it for that reason. I installed a bug shield and side window rain guards on my Sprinter and noticed an immediate hit on fuel mileage, which didn’t come as a surprise. The surprise was just how big a hit it was – more than half a mile per gallon. The Sprinter’s front end is pretty streamlined, so it didn’t take much to mess it up.

What do you guys think? Rain gutters look cool and all, but if they ruin a car's aerodynamics, then they may not be worth it.