Her insurance application will ask her to list all cars registered to her, and list all drivers. If she lies and you are in an accident, they will catch her lies and refuse to pay. She and you would be looking at possible insurance fraud. And the person you hit will be pushing the issue, believe me. Also, when your policy drops, your insurance company will notify the leasing company. They will then "activite" their own coverage (at a much higher rate than you are spending now) unless/until you provide proof to them that you have another policy in place. What proof would you be able to give them? Your girlfriends policy that doesn't even have your name as a listed driver? And if you did have an accident, what is your girlfriend going to say? That it was the first time you ever drove the car? Don't do this.