click to see sample footage (8mb quicktime)

features:
This rig securely positions a camera without interfering with operation of the car window.  This example has the camera positioned fairly close to the car.  Additional hardware can allow the camera to "float" further away.

The sample footage shows this rig running at 15, 30, 40, and 50mph as well as while cornering.

straight talk:
This rig provides a solid camera platform away from the car but isn't absolutely still.  Small vibrations will still get to an unsupported camera lens and appear in the frame.  Some cinematographers like a little bit of frame movement to give the scene a sense of motion (as opposed to a process shot on a blue screen stage.)  To reduce vibration further:

• Support the lens with a camera rods system.

• Travel at slower speeds.

• Add a wind cowling ahead of camera to direct the air stream away.

• Choose your roads carefully

• Rent a car trailer.

• Add a camera starter pin and grip head to the top of the camera.
• Build two outriggers each with a suction cup, grip head, and 24" steel rod.
• Slide a grip head onto one of the steel rods, attach open end of rod to start pin's grip head and secure the suction cup to the top of the vehicle in front of the camera.
• Attach open end of second rod through the grip head left sliding on the first rod and secure the suction cup to the vehicle at the side of the camera.  The two outrigger rods should now create a roughly 90-degree angle.  (The outrigger suction cups don't have to be positioned at front and side.  They may be rotated to any position about the camera as long as you maintain a 90-degree angle between them.)
• Adjust counter-tension between all elements to further reduce wobble.
• Double-check all connections for tightness and suction cups for adhesion.
• Grab the camera plate and rock it.  The entire vehicle should rock without the camera moving at all relative to the vehicle.
• Go shoot!