On mic placement, I usually just position it where my head would be, if I were sitting in the driver's seat, using a tripod. From the front to back sonic issues, having the mic simply in front of my face, while sitting in the driver's seat definitely alters the levels.

I don't think crossing over each speaker would prevent this ... it just has to do with fall-off after a few feet away from the source. The material in the car, echos, obstructions further cause this deviance. But, that's the fun of having audio in a mobile environment ;-)

I was using spectra lab, but the included radio shack file was for a different mic than the one on the SPL meter.

g