MPG rollercoaster

ok... for the past couple weeks (I drive a lot and fill up about every two days) I have been on the mpg rollercoaster. Appart from that when it idles it sometimes has a little shudder in th rpm's (sits low then it goes up by a bit and then returns to normal).

It's a 98 xj with 136K miles on it, just passed the smog check with flying colors (twice) and I got no clue what the problem is. I figure that the O2 sensor is good cause it had no problems with the smog test, and the tbody is clean (just recleaned it couple days ago to make sure). I use ussualy the same 3 gas stations, and my mpg varies from ~12 (one time as low as 10) to ~18 while before it was a stable 16 ~ 18mpg which I was happy with.

Any ideas what should I do next? Btw, it's a stick with 4.0 that runs fresh Mobil 1 every 3K, 4.11 R&P and 31" tires (and the pressure in tires is same as it always has been on the street)

You could be having problems with filling the tank. I have experienced this problem over time. The filler goes into the bottom half of the tank and I have read on here that was to prevent a fuel spill in an roll over.

The problem using different gas stations is the pressure the nozzle clicks off is not consistant. Also if the ground is level vs sloped had an effect.

IF possible try refueling at the same time of day - morning, mid morning, noon, afternoon, evening, etc - and use the same gas station, the same pump and park on the same side of the pump. See if that evens our your avg MPG.

Lets try to remove all the variables we can and see what your result is and filling from one gas pump may solve the problem.

Actually my fuel gauge is sensitive enough that I know when I fill it up right or not, so that is not really an issue. And the inconsistencies exist at all 3 gas stations, and I ussually fill up at the same time and it's never really been a problem before, so I doubt it's gas