Change your upstream o2 sensor. I gained 2mpg in town, not sure about hwy by doing mine. So now I get 17 instead of 15!

 

I was actually running a HHO cell and I've been perfecting and tinkering for many years. But when I had one running I noticed a HUGE power boost. Although, it was hard to gauge the fuel savings because I didn't get to run it through a full tank before the 25amp fuse blew. I'm working on a new design and I'll keep you posted.

 

There's a website called ecomodder.com with a whole list of mpg boosters. Its a forum just like this one but dedicated to better fuel economy. There's at least one xj and zj that I saw. Pretty neat website.

One thing I saw was actually a warm air intake that they all run. I don't understand why people think a cold air intake increases mpg. With a CAI you're getting more air, so the computer sends more gas to the innjectors to maintain air/fuel ratio. You get more power from a CAI, not better fuel economy.

Another thing to look into is a scangaugeII. It plugs into your OBDII connection and will give you instant mpg readout. It can also monitor lots of other things from the ECU. Will help with the led foot. And I'm pretty sure they can also read and save any codes that are thrown.

Might be worth looking into.