From what I got, if your box is:
28.5 x 13.5 x 32.5 / 1728 = 7.24cu'
I went with a port that is 24.5 w and 3 t:
(4 t and 28.5 w got messed up #'s. You always want to keep a 9:1 ratio to limit distortion.)
A port that is 28.75 L x 3 T x 24.5 W will tune to 39Hz with a peak of 7.8dB @ 46.5Hz.
(16.5 + 12.25) x 3.75 x (24.5 + 1.5) / 1728 = 1.62cu'
(Add 12.25 to the length for port correction factor. Add .75 to the h to compensate for the external height. and 1.5 for the external width.)
I don't know the sub displacement but I left .12 for it (Prob not enough but maybe close?):
5.5 + 1.62 = 7.12.....7.24 - 7.12 = .12
So: 5.5 air space, 1.62 port disp., .12 for sub disp.
Hope this helps, and, as always...have these #'s double checked.
2006 Jeep Liberty
Adire Audio 15" Brahma running off Cadence A7hc, Clarion hu, MB Quart 6.5s and 5.25s running off old school Punch 800a4
146+db @ 38hz with 1200w and a single 15"
Lv = [(14630000 x R^2) / (Fb^2 x Vb)] - 1.463R
*Vb = vol. in cu"
**R = sq.rt. A / Pi (for square/slot port)