Remove Excess Weight

"Avoid keeping unnecessary items in your vehicle, especially heavy ones. An extra 100 pounds in your vehicle could reduce your MPG by up to 2 percent. The reduction is based on the percentage of extra weight relative to the vehicle's weight and affects smaller vehicles more than larger ones."

Fuel Economy Benefit: 1–2%/100 lbs
Equivalent Gasoline Savings: $0.04–$0.08/gallon
The above quote came directly from fueleconomy.gov and is a very simple answer.

The key thing is to differentiate the effect of weight on stop-and-go driving vs. steady-state cruising.

"In steady-state cruising on level roads and with everything else being equal (e.g., a car made of aluminum vs. an indentical car made of steel), weight doesn't have much effect on mileage. Weight slightly increases the rolling resistance of the tires and slightly increases the drag of the bearings in the wheels, but at anything but very low speeds, these are minor contributors to overall vehicle drag - the big factor is aero resistance which is not affected by weight at all.

But in stop-and-go driving, weight has a big effect on mileage because you're constantly accelerating the weight up to speed, and then wasting this energy as heat when you apply the brakes."

This is a good explanation also from a different source and I did not include any with mathmatical formulas as there is not one little "super gas milage formula of if you add X weight you will decrease your gas milage by y" and also the math, while not difficult, is very complex and cumbersome.

The best rule of thumb for weight is this: Every 100 pounds added to your vehicle will drop your gas milage 1-2%. So you can see there is a drop in efficiency for adding the rack. It is slight but these things add up quick.

Now, I don't know what I said that would have caused you to post your comment. I did not say how much efficiency will be lost, just some will be lost. Most people seem to forget about weight and efficiency but remember the aerodynamic losses. I was just reminding them is all. If you think my posts are "the most ridiculous thing I've heard" or "gimme a freakin break" then you are entitled to your opinion but I usually don't post something unless I know a little about the subject.

One more thing: In the case of the gobi rack being discussed the loss due to weight will be greater than the aerodynamic losses. Why? I'll let you do the math and figure it out as your response to my result will be much more emotional than factual.