sealed box sizing for a single orion xtr 12s?

[moturbopar]

Hi, all I am trying to figure out what size box to build for my car. Orion recommends a 1.5 cu ft sealed enclosure. But my math shows that an ideal box is around 3 cu ft (but my math *****)! the speaker specs are
FS 23hz
Qms 9.05
Qes .47
Qts .44
SD sqin 78
VAS 7.92 cuft
the basket displacement is 209cid
Is there something I am missing or forgetting?

I am looking for bass quality, not SPL. Should I just stick with orions specs or go by the math?

Thanks
Shawn