There are several types of bicycle racks for cars.
Trunk mount, Roof mount, Spare Tire mount or Receiver Hitch mount. Trunk mount bicycle racks for cars are the most common.
Trunk Mount are usually cheaper than other types of racks.
Simple installation. Easy to store when not in use.
If your car is dirty and you put a trunk mount on, the dirt under the rack feet will act like sandpaper and ruin the paint.
Some Trunk Racks do not get the bike far enough from the car so the bike can rub on the car.
You cannot attach the upper rack straps to a glass window. If your car's back window opens and there is not an attachment point above that that is metal you cannot attach a trunk rack. Some of the older Camaros, Firebirds, Toyota 4 Runner.
If your exhaust pipe points directly at the tires on your bike it can overheat the tire and ruin it.
Many racks do not separate the bikes well and the bikes will rub the paint off of each other.
You cannot access the back of the vehicle when you have bicycles on the rack.
The bikes are well separated so they do not rub on each other or the car.
You can pull a trailer and the bikes will not be in the way.
The roof racks protect the finish on your car really well.
Often there are a lot of other accessories to carry canoes, skiis, snowboards etc. to fit the roof rack.
People have been known to forget the bikes are on the roof and run into the garage. This is really hard on the car, bike and sometimes the garage.
Usually Roof Racks are quite expensive. Often it will cost over $400 for a two bike roof rack.
Putting the bike on the roof can be a chore sometimes.
The extra wind drag from the bikes above the car will significantly affect your gas mileage.
Installing a roof rack can be a chore.
Sometimes they are the only rack that will fit the vehicle.
They do not fit many cars
Easy to install.
Less wind drag than Roof Racks.
The bikes are kept away from the car so they don't rub on the paint of the car.
The rack only touches the car by slipping into the Receiver and either bolted or pinned to the receiver.
Receiver racks are easy to move from vehicle to vehicle as long as the vehicle has the right size receiver. Check out my Receiver Hitch Tips page.
If your vehicle does not have a receiver it usually costs around $200 or more to have a reciever put on.
You cannot haul a trailer when your rack is on your car.
Like the Trunk rack and the Spare Tire Rack when your bikes are on the rack it is often hard to get into the back of the car.
The best selling Bicycle car racks are the trunk racks mainly because of the price. The Receiver hitch racks have become really popular in the last few years because they protect the vehicle, are easy to install, and will fit a lot of different vehicles.