Estimate fuel saved by aerodynamics.
Aerodynamic drag rises with the square of speed, so how much a drag improvement is worth depends entirely on how fast you drive. At 40 mph aerodynamic losses are only about a third of the total road load, with rolling resistance and drivetrain friction taking the rest; by 70 mph aero is around 60% and by 85 mph closer to 75%. That is why the economy gain is the drag reduction multiplied by aero's share of the load rather than the drag reduction itself: taking an empty roof rack off cuts drag by perhaps 8%, which at 70 mph is a real 5% improvement in mpg but at 30 mph is almost nothing. The fuel saving then follows from the difference between gallons burned before and after, not from applying the percentage to the fuel bill directly.
Aerodynamic Fuel Savings
Aero share = 0.35 + (speed - 40) x 0.008; economy gain% = drag reduction% x aero share; saving = (miles/old mpg - miles/new mpg) x fuel price
On the highway, yes — an empty crossbar rack typically costs 5-10% of your mpg at 70 mph. In town the effect is negligible, which is why the saving depends so strongly on your speed mix.
Because drag is only part of the total resistance. Even at 70 mph, roughly 40% of the load is tyres and drivetrain, and cutting drag does nothing for those.