If properly fitted , they will save the nose of the hood from rock chips.
Saying that years ago working at a Ford dealership in the Service Dept.
Ford had put out a TSB on causes for mpg lose.
Extended mirrors, running boards, BUG DEFLECTORS, lift kits, larger tires, removing lower valance piece under front bumper, mud flaps both front and or rear.
Can't recall the % each one cost mpg, but they took a truck on a closed track did several test runs for a base mpg. Then installed each item listed above then road test, removed installed next item road test (never installed lift kit and tested) and so on finally with all items listed (minus the lift kit) and came up with a loss of 2.9 mpg.