Car Racks significantly reduce car mileage.  It is amazing how many people drive around with ski or bike racks permanently attached to their car. 99% of the time, these car racks are empty.  Why not just remove them and install when they are needed.

No doubt convenience is the main reason.  Car racks interfere with aerodynamics.  We can hear a distinct noise whenever a rack is on our vehicle.  A site did a test and measured 12% less miles per gallon with an empty rack and 27% less mileage with a bike on the rack.  Placing a rack on the back of the car reduces mileage far less,  so consider a hitch mounted ski or bike rack.