Fire effects in CG can be quite convincing, but also time consuming. What I would suggest is setting a car on fire in a controlled envoronment, from a wreckers for example, and compositing it with the car you shot with your actor in it. That way you have live action fire as well as your actor, and control over how much fire you want in your shot because you have both plates to use.
Any additional fire can be created digitally (if you have the resources) and added in post to enhance your shot.
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