You don't need to align it to .707 if you do in-vehicle and anechoic. Although, if you know your alignment is, in fact, .707, you don't NEED to do the anechoic. In the past, when I cared to find it, I did in-vehicle SPL measurements and compared it to the anechoic. The largest desparity between the two is the point of the most "cabin gain".
If you do the math between say a hatch and a van, the difference between them is relatively small as far as sub bass wavelengths are concerned. For example... take 10ft for a hatch or whatever and even 20ft for a van, maybe. 1/4 wave of 10ft is 28hz. 1/4 wave for 20ft is half that, so 14hz. But both of those are too low for SPL purposes, so take the half wave. That's 56hz and 28hz. 56hz is lovely, but 28 is still too **** low. So take the full wave. Right back to 56hz. So seemingly... a hatch and van will both benefit from 56hz.
In my world, I'm much more concerned with tuning for impedance rise than "resonant frequency". The car is going to peak where it wants to. I can put a daily in my CRX and you'll see two distinct SPL peaks. One near tuning and one near where I know what the car wants. Regardless of tuning, I'll see that bump. The trick is not to force the note there, but to massage the impedence rise to it's lowest point at or near that note. That might be by tuning above that note. It may be on the nose. And I honetly can't tell you what my SPL boxes are "tuned" to because I don't tune them. I test them. It's tuned to where it's the loudest.
