Roof rack + MPG

I'm curious about roof racks and the loss of MPG.

Is it due to a heavy roof rock?

or due to wind resistance? (no fairing on the front to help push the wind over)

a combination of the two?

Weight will always reduce your MPG, but I believe wind resistance will reduce your MPG by a greater margin. This is easily tested by simply adding comparable weight to the inside of your Jeep for a few tanks and seeing if you have any milage change. My bet is you will not notice any change due to driving variables and the small amount of weight you are adding for a typical rack (<40 lbs).

My ConnFerr rack is responsible for just under 1 MPG loss. The wind noise is very noticable when it is installed so I'm convinced the added resistance on top of my toaster-shaped vehicle is adversely affecting the already crappy mileage. For this reason, as well as keeping a lower CG, is why I run my spare inside my cargo area. The rack looks cool when it is installed, though.

My kid's rack (green XJ) has considerably less bracing and has less wind noise and resistance. Choose a low profile rack that is sturdy, but made with the lightest and thinnest materials possible.

slope the front... like this

http://www.vii.in/Images/Nissan-Xterra/Nissan-Xterra-3.jpg

my boss took his roof rails off of his Volvo wagon to drive to NH from DE 12hrs and he found a 2mpg difference. on a jeep i would think you get the same with the cross bars. i modded that back of the rail so that i can just remove the bars. but i never did a comparison but with a rack i would bet its 2mpg worst then having bars on. maybe 4 above what you would get with a clean roof. and i doubt the front fairing does much but swirl the air. its still going to hit the sides and back bars get disturbed and slow you down. i don't think you would notice the weight of the rack

I installed a ProtoFab roof rack on my '07 with 4 rectangular lights. I very promptly lost 4 MPG on the highway...

We drive brick walls, aerodynamically speaking. So if you increase the height of the brick, the resistance goes up and the mileage goes down.

my old con ferr killed my daily driving and highway mpg by a couple mpg's... plus it was heavyyyy... low profile is the way to go... if you can help it do not put the tire up there!

The rack is being made out of a futon frame and EMT conduit. It'll be light. I plan on having 6" Hellas up top or get a smaller diameter light to help w/the height of it and have a fairing in front to help push the wind over top and have it as close to the roof as possible to keep the LCOG and not stick up super high.

I plan on making a quick disconnect for the lights so if I don't need the rack I can take it off easy. The stuff going up top wont be anything drastically heavy, camping supplies and such. Eventually I'd like to get a M416 trailer as my Jeep will be more of an overland/adventure rig and would like to have a trailer to help haul stuff for longer trips.

I just wanted to get input on what effects the MPG... I figure with such a light rack and something in the front to push air the MPG loss might not be as bad or necessary to take on/off.

Eventually I'll have the spare on a Detours tailbone.

Air resistance is the main culprit in mileage loss with a roof rack. A really heavy rack weighs less then a person, and your mileage doesn't vary noticeably when by adding a passenger, does it?
Air drag goes up at the square of velocity, or in English, double your speed, quadruple your drag. Our bricks have pretty high drag to begin with. Add monkey bars to the top, and it goes up even further. Add 4 cup saucers(lights)or pie plates(big lights)and you might be able to add a full point to your base drag coefficient.

If you want to make a low drag rack, figure out a way to press the tubes into an oval, and orient the long axis front to back. Oval tube has about 2/3s the drag of similar sized round tube.

As for the stock cross bars, I was wondering about that myself. I'm making a long highway trip next month and was planning to pull them off and see if it made any difference.

Also ... you can get a piece of smoke-gray plexiglass and fab an angled wind fairing, which should cut down on the wind noise and increase your MPG.