It’s debatable whether it uses more fuel to drive with your windows down on the highway or with the windows rolled up and the air conditioner on. In tests by Consumer Reports and Edmunds.com, it depended on the vehicle and speed. At 65 mph, using the air conditioner was slightly more draining. At higher speeds, it probably wouldn’t be.
In any case, the difference is small either way and one thing isn’t debatable. Driving with the windows down on the highway is noisy, distracting and possibly dangerous. Road debris can enter the car and the wind can catch items in the vehicle and blow them around. Given that, just roll up your windows and enjoy the cool air.