Welcome to the forum, zjrog -
Looking forward to watching your progress. If you're serious - put it on display so the rest of us can follow along. (IE - make a
garage entry for your Jeep, and fill out the fuel log.)
Roof racks: there are no hard & fast numbers, because the style differs from vehicle to vehicle. Aftermarket racks are almost always going to be worse than OEM. The *least* harmful aftermarket rack tested and mentioned in Hucho's
Aerodynamics of Road Vehicles adds 0.10 to the Cd, which is enormous. Even if your OEM rack is a third of that, it's worth taking off.
When
I tested the aero penalty of aftermarket racks empty vs. racks + bicycle on my car, my mileage fell by an astonishing 12.7% - and that's with nothing on the racks.
You'll note that GM has left the racks off its hybrid Tahoe.
I'd say: take 'em off. They're certainly not helping.
cheers-
Darin