One thing you fail to consider is the optimal operating temp of your engine. When cooler than normal, it's going to use more fuel. My advice, buy a thermostat and gasket (and a couple gallons of coolant) and keep it handy while running without one. That way if it does start to give you problems, you can put it in regardless of where you are.

 

This thread brings the Friday morning fun!

 

Another thing to consider is that XJs are run by a computer, which regulates fuel, timing, etc., by the engine temp.
I have run my 38 Plymouth flathead 26 years without a thermostat, but any vehicles I have, run by a computer, I run a therostat.

 

I can honestly say, I have never used my XJ to cook bacon, but,,,,
I have wrapped a big old tater with aluminum foil, placed on the engine for a few hours while mudden in the hills, and had a nice lunch.

 

+1 better than the funnies

 

I stand by my statement. if you remove the thermostat the engine may never get up to temp. when you have a thermostat in there, and it opens, it will stay open until the temperature drops below its rated temp. It can run cooler without one yes. but if you have to take it out you have another issue in your cooling system you need to address.
and your engine will run rich resulting in worse fuel economy. newer obd2 xjs would likely throw a code too..

 

X2