The rocketbox is your best bet in saving MPGs while carrying skis and maintaining cargo room inside the vehicle. This type of rack works best because it protects your equipment while driving, is aerodynamic and when you take the box off of your truck (very easy) you can keep your equipment in the box, saving storage space in your garage.
Open racks on the top of the vehicle kill gas mileage while still leaving your equipment outside of the vehicle. Second best option and works well as long as you maintain your equipment well.
The rear mounted (or hitch mounted) ski racks are, in my opinion, horrible. Your equipment gets all the salt and road grime from your tires while putting all your skis in danger of damage. Even open roof mounted is much better than these unless you like replacing binding bolts, and edge tuning every few days of skiing. These should be used as a last resort. I had to deal with these kinds of racks because I drove a soft top jeep wrangler for years while I was a ski instructor. Almost no room inside and no way to mount it to the roof.
Well back to your question. I haven't seen much of a hit in gas mileage, maybe 1-2 MPGs while driving with the ski box on my truck. I normal drive a little faster though when I have it on cause I want to get to the damn mountain.
