I'd say it's more dependent on the acoustic effects of the car and the rest of the system - specifically how low you can cross the mid/midbass.
My electrical filter is a L-R 2nd order at 70hz. My enclosure is a 4th order so I also have a natural acoustic LP filter from that. I have a nodal cancellation between about 60-70hz so there's a huge dip there. This combined with the electrical filter gives be an effective acoustic crossover of about 60hz 24db/oct.