resonant frequency is the frequency something naturally reacts to (vibrates, what frequency a sub wants to move). I don't know exactly how accurate this is but Soudnstream made there SPL mules to resonate at 60hz, that to 120hz is supposed to be where cars naturally resonate. Most bass boosts are set to around 40hz which is were cars are supposed to have trouble reproducing, but the main thing here is people usually don't want cars to resonate (resonating causes rattles) so I think what you might mean is what frequency will cancel out the least at the drivers seat.... dunno just rambling hope it helps. PS I have also heard that most competitors run higher frequencies when they hit the really big numbers and most SPL boxes are tuned high like 50 to 100hz but most regular boxes are tuned around 35
565/the longest length in your car(in feet)= resonant freq in hz. So if you measure from top of back window to front foot well and it's 9'6" then divide 565 by 9.5 to get 59.5hz.