99' Grand Cherokee Laredo gas economy?

I bought this car used a few months ago with about 160,000 miles on it. It's a straight 6 engine. I have been looking at my fuel economy via the overhead console and it hasn't left 12 since I purchased the car. I have driven it on the highway for a short period of time and even then the mileage didn't move much. I'm just wondering if this is about what I should be getting on this car model or if something is wrong. I have considered running an engine cleaner through the fuel tank and seeing how much it affects it but figured I would get your opinions on it.

well, I have an 89 XJ with the same engine, I have upgraded the exhaust w/ headers and cat back exhaust, installed # 19 injectors, lapped the valves and replaced the valve seals, installed heavy duty H-gasket, and converted to open coolant system. My gear train is 3.55 open diffs. Currently running 87 octane I have only been getting about 12-15 mpg, mostly 12 though. Thats running it 22 miles one way to work. I have 235/75 R 15's for tires set at 33 psi. SO what you see in milage is about right. One thing though, you could try testing the milage by putting in about 10 gallons of fuel and reset your trip, see how far you go and do the math. MIlage has allot to due with your personal driving techniques, hard on the accel on take off, or ease into it. Stop and go traffic will kill your MPG.
If everything is stock, I would recommend getting a good injector flush done, usually a reputible garage can do it. I recommend Wynns Kit, its a 3 part system, one for the injectors, one for the fuel tank, and another to clean the carbon deposits off your throttle plate and intake.

Not to say gtmud's mpg calculation is wrong, but an easier way is to fill the tank, drive for at least 1/2 tank, keeping track of mileage, then refill and divide the miles by the gallons.

My wife and I had a Cherokee and got 22 or so on the highway, although it would drop to maybe 15 in town. Another thing that may affect the mileage is if the catalytic converter has failed in the rich position. If so, the inside of your exhaust will be black and wet and probably smell like gas.