I lose about 10% gas mileage with two singles and a tandem on my roof rack. Many years ago I did the garage goof with a bike on a roof rack. Damaged the car roof, a foot of the rack, the garage, but not the bike. I was also rear-ended once with a bike on a trunk/bumper rack. Totalled the bike. I don't have a garage now. One suggestion I've read here, if you have a roof rack, is to put the garage remote in the seat bag of the bike.