Miles per tank has no meaning at all, sort of like ants per picnic. The gas gauge is not calibrated, and the readout is not linear, it is just a more or less indicator.
Calculate your actual Miles Per Gallon, and then use the Google to compare your results to other XJ owners with similar modifications. If necessary, adjust your MPG figures for non-stock size tires and/or gears.
To increase your MPG's, do all the normal stuff, tune-up, make sure your tires are properly inflated, and perform any necessary maintenance/replacement required to the catalytic converter, O2 sensors, and other engine sensors.
Progressive Liberalism: Bringing you new ideas so wonderful, they have to include mandatory participation ......
Originally Posted by Ronald W. Reagan: Government is not the solution to our problem; Government is the problem.