You aim at her head and pull the trigger..
*CRACK*
Her lifeless body slumps over half onto the passenger seat, held up only by the seatbelt. You unbuckle the belt quickly and lift her over onto the passenger seat. You put the seatbelt on her, get in the drivers seat, and drive away checking the rear-view mirror for witnesses. It's all quiet.
You drive a few miles before pulling up off the road into a dead area of woodland. You get out, leaving the engine running, and pull the body from the passenger's side, letting it fall to the floor. You close the passenger side door, run around the car, get back into the drivers side, and drive off. Destination; airport, no stops.