It depends, some vehicles it's less than 6% increase for some of the rack products but other vehicles with high load type racks it can be as much as 25% increase in fuel burn. AC is usually cheaper than driving with the window down too... Another example is a family down the road a bit actually dropped the roof racks when they were carting a handful of mountain bikes and went to a trailer for them. Apparently paid for the trailer in the first year with the fuel savings? Basically bolted the racks onto the trailer, Mum really appreciated that part of the change as less lifting onto the roof of the SUV.
But as far as versatility roof racks can be really useful, especially if you have the odd need to cart long loads. My experience of them is the Thule products are well thought out and do the job, but you pay for that.
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