That's the thing, Man... She's still going to find out what's going on. You think you dumping her isn't going to raise even more questions when she finds out there was some random skank in her car with you?
If you want to be deceptive... Do it. But preemptively dumping her is almost hilariously counter-intuitive.
If keeping her is your sole focus, then also keep it simple... Tell her a co-worker needed a ride home, and get out ahead of the situation by bringing up this detail before she finds out. For good measure, mention that she may want to file a claim too, and say something about how she said she may need to visit a chiropractor... making it seem above board. And putting your girl on the defensive.
Realize nothing you say will be totally believable, but that's just the nature of this situation. Get a story and stick to it... Unless there's a clear smoking gun, women will rarely leave even if a situation seems shady.
Good luck.