Just to kind of list things that an hit mpg in one place and assuming your new ride has already had a DOCUMENTED timing belt job done, may have been mentioned in the list but I did not notice.
1. winter fuel - I've seen mention of a 10% mpg hit or more especially without using a winter fuel treatment like the appropriate PowerService.
2. bad thermostat and yours is bad - I've personally seen at least a 10%+ mpg hit
3. dragging front brakes - another 10-20% mpg hit. Easy to check - with cold Jeep on level or slight slope does it roll easily? After a brief drive with essentially no brake use are the wheels/rotor really hot, be careful when checking.
4. viscous heater until engine is warm might cost 1mpg.
5. cold weather and short drives (e.g. anything under say 15 miles even with a functional tstat) - I've seen a hit of 3-4mpg
6. cold drive train fluids - for example tranny won't shift into higher gears at all unless tranny fluid is above a set temp.
That's not to mention tire pressure, wind, and terrain.

I'm assuming your mpg is based on hand calculation and a GPS correction of odometer reading as the EVIC is notorious for being off but usually on the high side and odometer can be +-5% with stock tires and still meet spec.

Personally I would avoid the inline tstat option but it's ok for short term use.

If you haven't done anything about the tstat by the end of March you can have my used OEM (14k miles) for the cost of shipping. Stoutdog gets the TDF one and I get Kap's from stoutdog.

..wow , 480.. Seems like an inline might not be safe either. Id rather not risk it.

I can tell you that this jeep does run very cool, even if its been running a while. I thought this was normal. Of the 4 sections on the temp gauge I have never seen the temp go higher than the middle of the "1st section". It also can take 4~5 miles before I get any heat from the heater. Is changing the thermostat difficult?? would you say it was sticking or not working?? Do I have to take off 1/2 the engine to get at it? This engine does run very cool.

A failed (stick open) thermostat will absolutely devastate your MPG. Think about it, your engine cannot get up to temp to where it is designed to operate efficiently. All of the other factors mentioned are valid, but this one is guaranteed to cause poor MPG.

If money is tight and you need a $10 band-aid option, see the link below. Install into the upper radiator hose with the "bulb" facing the engine as close to the inlet as possible. Then calculate your MPG over the course of a full tank of fuel and see how it compares.

Stant 13519 Thermostat - 195 Degrees Fahrenheit

Correct - low to really low ambients can result in reduced fuel economy
- low temp air is dense air - dense air is more difficult to pump\compress, which results in more power lost to overhead
- tailpipe emission would appear normal as engine is actually burning the excess fuel to maintain function
- can also result in incomplete combustion with less BTU per injected volume = reduced power = more fuel consumed per volume of work
- tailpipe emission ranging from white vapors, where fuel did not reach combustion temperature, to gray to black haze, where fuel reached combustion temperature but flame went out are also normal during winter climes

This is where it would be better to have a steel air filter case with temperature-controlled valved ducting over to an exhaust pipe stove, such that intake air would be preheated in winter ambients, resulting in reduced pumping effort and improved fuel economy

yeah, if you do use the band aid, make sure you drill a tiny (about 1/16 should do) hole in it if possible (not sure with that style as I have the big clam shell inline in mine that houses a regular stat), as that will help with hot spots, but you could get unlucky...

Pulling intake air across a stove would ease engine overhead in low temperatures, similar to CUMMINS intake air heater, which, if you observe the ammeter\voltmeter, you can see that intake heater cycles quite often after engine starts and is running - and, their CAC is much larger than the KJ's

The CAC is intended for those climes where ambients are 100* and charge air is 250*^

Ambient temps below 30* also make the turbocharger work harder where the compressor blades are cutting thicker air - takes more power to make Boost in low temps

Pulling intake air across a stove would ease engine overhead in low temperatures, similar to CUMMINS intake air heater, which, if you observe the ammeter\voltmeter, you can see that intake heater cycles quite often after engine starts and is running - and, their CAC is much larger than the KJ's

Any warpage on the exhaust side?