[Fat Charlie]

You've got a 4.0 trying to shove power through an AW4 in order to move a literal brick through the air. There is so much inefficient slop in that chain, do you really think a tweak to fuel delivery is going to be noticeable in mpg once it gets through the engine, an anchormatic transmission, the driveline and tires and the aero losses? You'll notice more of an improvement by rearranging the deck chairs on the Titanic than upgrading an XJ's fuel filter.

If you got an improvement, it was because you had an injector that needed replacing. 1997 model year had OBDII. Get an SG or UG and you can see real numbers.