Depends upon your car and how aerodynamic it is. If you're talking a squarish SUV or minivan, probably not much; if you're talking a Prius or Honda Insight, you could see a considerable drop.

I drive a Prius and mileage dropped from 55mpg to around 40mpg with the rack and 2 bikes. Just the presence of the rack dropped mileage. We've since learned how to get both bikes inside the car if we're transporting for any distance.

But, we never had any issues with the Saris Bones slipping during transit. Yes, the straps loosen over time, but no problems with sudden slipping like other racks in the past.