Someone on a different forum brought up something about the car that makes sense to me - that Casey abandoned the car simply because of the smell, not because it ran out of gas.
If the car had just run out of gas, when Casey called Tony to come and pick her up, why would she lie and say her father was going to pick up the car because it was broken? Why not ask him to bring her some gas so that she could continue using the car? And then why would she leave her purse in the car?
Perhaps she was in the check cashing place trying to cash one of Amy's checks. When she came out people were outside the car complaining about the odor loudly. She doesn't want any of the people to know it's her car. So she uses her cellphone that's in her pocket (or her hand) and calls Tony, has him pick her up far enough away from the car that he wouldn't smell it.
This would explain why she a) didn't have Tony bring some gas and b) why her purse was left in the car.