I would tell her that she's straining your budget and that four months in, she needs to be making contributions because you simply cannot afford to provide for her. Let her know that if she cannot pay for a third of food, fill your gas tank every other week (or however often would be fair) and start paying 1/3 of the rent and utilities, she will need to move out. If you're driving her to work, she's making an income and is taking advantage.
Let her know that you hope she will not confuse what you'd like to do as a friend, and what you must do because of your own financial limitations - you cannot afford to support anyone but yourself.
If she's offended, that's on her. As long as you don't talk about what a jerk and freeloader she is, and just talk to her about the financial need, she should be appreciative and glad to kick in. If she can't or cries or claims she can't afford it, you'll have to hold your ground and say, "I hope you recognize I've done all I can for you, but this isn't sustainable. Please make other arrangements by the end of the week because I just can't afford this arrangement".
Unfortunately, if she doesn't leave willingly, you might have to legally evict her.