brian
it will all depend on what equipment u are using and what u do to your car,
if u stick two 15s in the boot and leave it at that then u set it up to your frequecy, but if u add things like dynamat then your resonate frequency will be different, also applicable for box sizes air space,
every car is different, u may have seen some spl vehicles lining their roofs with concrete this is because concrete has a very low resonate frequency. how far do u want to go?
bear in mind the SPL Riles have changed this year?
feel free to ask any qs?
Tom