Just looking around at stuff online today and wanted to know how much space I have in the back of my X. Found an 18" sub that says ported it needs 7.5 cubes...is that doable?

First thing I would do is see if the manufacturer has a recommended ported box dimension spec. A box can be built with the correct volume but improper dimensions. If you can find the recommended dimensions, see if you have the room for a box that size.

Also, I hope you realize that you'll basically lose the entire "bed" of the X

I am sure its doable one way or another. Probably would end up basically being a wall from the ceiling to floor.

Personally I wouldnt do it unless its strictly a show truck.

I agree with ProlificX about not doing it unless it's a show truck. You throw a JL W7 of ANY size in your X and it'll shake the paint off of it. Even the 8W7 is a beast. When it comes to speakers (subs especially), bigger is NOT always better. Find a quality sub with HIGH excursion and you'll be very happy.

Anything is possible with enough thought put into it...And fiber fill added to the box will mimic the subs being in a larger box (I can explain this in further if you want), even if you cant build the exact box size the subs recommend..

I've seen 2 -15's in a corolla so Im sure you can get 1-18 in an Xterra Just dont expect it to sound good unless you soundproof the heck out of the rear hatch area..

I don't understand why you would want to put an 18" in there. What kind of music do you listen to? If you listen to anything where you want tight base, then you wouldn't want to go bigger than 10". Also, remember...if you want that 18" to be anywhere near tight and efficient, then you better have a big amp to power it. An 18" would be great if all you listend to was Rap and Big Base music.