How do I tell her that her car smells without offending?

I carpool with this lady, and she's really nice. But in the last few months, I've been noticing a sour stench coming from her car, and when I ride with her, this smell stays with me all day and sticks to my clothes. It makes me miserable. I know I should just be straightforward and tell her, but I can't find a way to phrase it without hurting or offending her. Help? Maybe I should just look at other transportation alternatives and forget about it altogether. Only legit advice please. Thanks!