Okay so i posted another thread about my bad fuel economy. i get about 12.7 MPG. i found a vacuum leak. The line running from the cruise control box to the vacuum booster pill looking thing. could that 1 leak have caused the worse MPG?

 

It could contribute to it.

 

Where there was one vac leak there may be others. The vac reserve "pill" behind the front bumper is best if relocated under the coolant tank. Yank out all the excess crappy plastic vac line that runs up there and you'll eliminate a big source of leaks. Also check your intake manifold bolts. They tend to work loose over time and cause big vacuum leaks. O2 sensors get old and give poor readings to he computer which can affect mileage big time.
Best of luck!

 

Excellent idea, Nick. I've moved the "pills" on all my XJs.

Here's a photo for the OP.