one 12, two 10's, or three 8's?

Im putting a new sub enclosure into my truck to get me some more bass. Im gonna have around a 1.5 feet of air space. Should i go with one 12, 2 10's, or 3 8's.  Im not sure which way im wanting to go but im gonna go with the same brand and same model with which ever setup i achieve

Is this a reg cab truck or xtra cab? What kind of tunes do you play and are you trying to blow the doors off of the neighbours or do you want good sound? Sometimes the choice of subs is limited in reg cab pickups due to the mounting depth of the sub. You do not want the sub hitting the back of a seat. I personally like the sound of a good 10" sub in a ported enclosure (MB Quart did this for me) or dual 10" in a sealed enclosure (what I currently use).

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Top Secret, I can tell you but then my wife will kill me.

reg cab ranger im interested in both sq and spl, more sq but i still want it to be loud enough

Forget the 8's IMO.  The quality of the speaker will be the most important factor in how many and what size sub you need.  The direction your box fires will change the sound to.  Sealed takes more power to achieve same spl as ported, but tend to have better SQ.  I have ran both, and currently run a bandpass in one car, and a ported in the other, several of my former systems had sealed boxes.  I used to run 2 12's but as I have gotten older (27 now) my taste have changed and i find that a single 10 can provide all the boom or punch I need.  You may be surprised how hard one good ten can hit, try to find one and give it a listen.  You have some requirements you will have to meet, mainly mounting depth, so that will be your guide.  Several good subs are out there.  JL Audio, Kicker, MB Quart, Sony.  Ok, maybe not Sony.  As far as the 12 goes...  A good 10" will outperform a lame 12" in all aspects but SPL.  If you are considering a 10 or 12 of equal quality, then go for the 12, you will always have the overhead if you need it.  But if its a 10 of high quality, or a 12 of lesser quality, go for the 10.  I installed a 10w3 in a sealed box with a US Amp (250w rms supplied to the sub) in a Fullsize Chevy Regular Cab, it pounded, boomed and felt great on your back : )  On a side note, hope you dont neglect the rest of the sound spectrum!  Nothing complements good bass like nice, sweet mids and highs.

im puttin a set of 5x7 components in, would 2 10w0's be nice

Sounds nice. JL subs are my sub of choice, some may say over priced, but few argue against the quality of the product.  By chance, are you planning to get the w0's off the forum? I just read a post were you inquired about them.  What 5x7's are you looking at?  Try to power those with something other then the deck to take full advantage of your upgrade.  I recommend a good 4 channel, bridge the rear outputs for the subs.

the components are either gonna be a set of infinity reference 6800cs or a set of JBL GTO8606c, prolly the infinitys, and yeah those jl on the forum are what i was looking at.  Im prolly gonna sell my planet audio 150.2 for a 4 channel