If you really can’t just say no, turn it into a logistic nightmare she will want out of. Tell her now that her car is fixed you are looking forward to taking turns driving! Say a week on/week off works for you and she can take the first week. Make up a schedule and email it to her. Be EXCITED. If she doesn’t immediately back out, be late the first day she picks you up. Eat something messy in her car while she drives. On your week, make a ton of random stops for errands on the way and become unreliable. Pick her up at the gym? Doesn’t she have a guest pass? Maybe you could just join her!! She will all the sudden not need to car pool.