Scratching my head on this one...
 

Alright, here are my jeep specs: 5.13 gear set. Dana 44 front and rear
35 Nitto Trail Graps
OME HD coils
Shrockwork Mid-Width bumper
1.5" spidertrax spacer
1350 J. E. Reel Front Drive shaft
RR Snorkel

I get around 14 mpg most days running hwy between home and work (30 miles one way) at about 65-67mph. This is with my spare off.

Now if I put my spare on, I can run about 70mph but my mpg go up... Is this normal???

The spare helps with the aero drag of the square shape back. Have you ever seen the fairings added to the back of freight vans? They help with the turbulent air flow.

Interesting... I have a slant back soft top. Guess it's still helping with the aero.