Rear Mounted Spare
 

How does a rear mounted spare affect aero on an already boxy/bluff tailed suv? Will it if mounted in the area not affected by the airflow coming around the vehicle from the front?

For an XJ I don't think it would make a difference in aero at all.

spare
 

ditto winkosmosis' appraisal

While I've been stuck without my Jeep, I think I've managed another ecomod: a removable canvas framed boattail. If the drawings and design idea's I have work out, it should be able to mount into my 2 inch reciever and be able to stow when not on long highway trips. Granted, I won't be able to touch this project for another couple of months!

Im always dreaming like that haha I have TONS of projects... tons! (but no time to do them lol)

To answer your q, yes it would not matter since its not anywhere near airflow.

Based on what I've read, the boattail doesn't have to be SUPER strong, just structually sound and as rigid as possible, correct? My thoughts are if built correctly, it should be able to be completed under say 60 lbs or so including the steel hitch mount? Granted it's a custom application so far, but who's to say the general principals of the design couldn't be applied to other bluff-arsed suvs and hatches with hitch recievers?

Skunk Works
 

Yeah,just strong enough is the ticket.That was Clarence"Kelly" Johnson's strong suit at Lockheed's Skunk Works.
Model airplane technology would suffice nicely.Glass over foam another path.
The receiver-hitch route gets you a couple of feet extra length out of the shoot.Great for any SUV!.And easy on and off.
If you can begin your taper over the back portion of the body even better.

I have an idea for the rear of the XJ that takes advantage of the straight body lines. Put flaps on all 4 rear edges, each slightly angled inward, curved if possible. With about a foot of length you could decrease the cross section a decent amount.
The flaps can be made of aluminum and attached with double sided tape.

I agree completely on the hitch reciever route, as that's part of the original design, but I'm really leaning toward a non-permiable canvas-over-collapsable frame design. I have provisions for the rear lighting and licence plate relocation, though I haven't decided on how feasable (or difficult) a rear window (to see the tailgaters) would be.

By this, do you mean begin the taper before reaching the bumper (for which the answer is yes), or cut a chunk out of the vehicle to begin the taper process (which I can assure you will NOT be happening :D)

I fully intend to do just that (see above), except with more than a foot of length. If I could get some help from any fellow cherokee owner (any year XJ body style), could someone PM me or reply to this with close dimensions of the rear hatch? I'm looking for bumper to roofline and from one outer taillight edge to the other to refine some measurements thanks! ;)

I'll measure when it's not freaking cold outside. I can get the angle of the glass too