get a cheap Radio shack SPL meter, BASS CD test CD with known freq on each track. Step One outside the car. Set the volume to one spot. play each freq and note the SPL @ 1 meter. So you get a curve on a graph. Like 20hz- 90db,25hz-92db,30hz-93db. Now put the sub in your car and keep the volume the same. Do the testing again. You will see the peak. There might be more than 1. After your done take the incar response and deduct the out of car response from it . Like so 20hz-97bd[incar] - 20hz-90bd[out] give you 7db of transfer function boost
