I just buy a gallon of 100% concentrated coolant every time I have to break into a cooling system. The 50/50 costs almost as much as full strength, but I have a reverse osmosis unit at my sink that I dilute it with. So it's about half the cost of 50/50 if you do the math. Anyway, just grab a gallon of ethylene glycol (green) coolant and fill from the radiator cap. Leave the cap off, let it run for a couple minutes, and periodically squeeze (slowly) the lower hose. When you quit getting bubbles, cap it off and you should be good.

Or blow a buck on a gallon of distilled water....end up with 2 gallons of coolant for 2 bucks more than 1 gallon 50/50 premix

parking with the nose high up (if possible), with the rad cap off overnight seems to negate the need for a lot of faffing around with bleeding the cooling system

I do run the motor a couple of times to circulate the coolant before leaving it that way

I replaced the thermostat, and after a 30-minute drive going 30-40 mph, it still would not get up to temp. Am I not driving it long enough? I also put the old one in boiling water, but since my little brothers apparently broke the thermometer, I was not able to determine what temp it opens, but it did not seem that it opens at an absurdly low temp. Does this mean my coolant temp sensor is broken? I don't think the gauge is broken since I am getting bad gas mileage.

We need data. Like the reading from the thermostat housing with an infra-red tester.

Or, grab the upper hose with our hand. At 195* you can't hold on long!!

The upper hose was definitely hot. It seemed to be about the same temp as the one that goes to the heater

an infra-red gun is a cheap and useful tool

I will repeat, that in my warm climate, my XJs typically runs at 180-210F in warm weather (75F+)

but if it ever gets cold (when I travel in the high country in winter, where the temp is only 40-50F), my gauge never gets anywhere near this if I am travelling at normal speeds

I would say we have a climate similar to areas of California, so not super hot

It takes about 30min to get fully "hot"..maybe 1 hour to reach maximal temps, depending on load and ambient temp, same as other largish all-iron engines I have had

See Post 8.

OK, let me try. This all started with "noticed that the temp gauge did not go all the way up to 210 like it usually did when I drove it in the summer." Later it was mentioned that fuel economy was bad so probably seeing low engine temps.
Now:

1) I have no idea what "not all the way "means. Like 10 deg less or 50 deg less.
2) there are about 500 things that can cause poor FE on a XJ.
3) ambient cold soak temp does make a difference re how long it takes to get to a stable temp. When it's -10 like it was here the other morning it takes longer than if it's +45.....but a half hour at 30 mph is way too long.

If it were mine and I was worried about it (I'm not, mine sits very nicely just below 210 on the gauge), keeping in mind these gauges/senders are not NBS certified, I would do all of:

1) make sure the elect aux fan isn't running when it shouldn't be.
2) check the temp around the sensor with an IR tool (not harbor fright ) and compare to gauge reading
3) test thermostat, if someone broke your thermometer wack him upside the head and get another one

4) take out sender, heat it up in your water bath with your thermometer in it and compare output to specs
.......

It's more like 50 degrees less. The gauge temp is about 160-170. Maybe I can try testing the thermostat in the oven instead. Would it still need to be immersed in water? I also unplugged the aux fan to make sure it was not on.

Also, is there a way I can put a voltmeter on the coolant temp sensor to get readings and compare to the dash?

1) ok, 50 deg matters if the gauge reading is accurate.
2) no, an oven won't work at all, their temp control is useless for what we are trying to do.
3l extra points awarded for disconnecting the elect fan.
4) there are not many things that can cause what you are seeing. Either the gauge reading is off or the thermostat is screwy (or you live in Alaska). To settle once and for all the gauge issue I would take the sender out, leaving it connected to the wiring harness, heat up a pan of water on the stove, stick the sender into the pan of water (remember you'll need to rig up a ground from the sensor body to engine ground) , put your new thermometer into the pan, turn the vehicle to "on" not "start" and compare the gauge reading to thermometer. By now the water will be cooling off so
take your propane torch along so you can heat up the water to get a number of temp points (up to 212...depending on your elevation of course ;-l) this will check the sender, the wiring, and the gauge. If that checks out ok you can then scratch all that stuff off your list and try to figure out why the thing is over cooling

By sender do you mean the coolant temperature sensor?

Yeah, the one that feeds the gauge. I have lost track of which MY you have vs whether it has one or 2 temp senders. Turn the vehicle to on . Find a temp sender and disconnect the wire. When you find the one where disconnecting it's wire kills the gauge you got it ( that's the crude but effective method)
you need to remember with all this stuff we are dealing with vehicles 25+ years old. ANYTHING could be messed up. You could get the specs to test the sensor out of the shop manual, but If it tests OK that still leaves the wiring and the gauge. This way you test the entire temp monitoring system; if it works all that stuff is OK and you can move on, if it doesn't then you can start diagnosing the system to figure out which part of it is messing up.

My manual says when the engine is cold, resistance should be 7000 ohms, and at operating temp, it should be less than 1000 ohms. Im gonna test that when I go out today. I also notice my gauge goes up high when I start it, I assume it's to test the gauge. Is there a certain position it should end in when its doing that to see if the gauge is whack? It seems now its at 2 o clock.

So, cold resistance of the coolant temp sensor was 35k ohms, and hot was about 1k. It was about 35 degrees outside, so maybe that had a factor. Does this mean its broken?