My .02 cents is that you want to keep the engine running as cool as possible. Now, you have to balance that against what type of systems you have that are temp sensitive. If you have a newer truck with all the computer wizardry, you want to stay at factory specs. The computer is looking for specific parameters and tries to compensate if it does not see those parameters. If you have an old beater like mine with no computer stuff, then you'll want to put in the coldest thermostat available as long as you generate enough heat to defrost the windows.
I've read that putting in a cold thermostat (165-deg. F) on a computer vehicle will enrichen the mixture as the computer sees a "cold start" condition and will enrich the fuel curve, a "pull the choke" if you will; in fact, it is one of the 5.0 Mustang guys cheap go-fast tricks. I did it on my '91 300-6, and it didn't make a bit of difference other than bump the idle 200 RPM and make the temp needle barely move off its low peg.
I also pull a 6000-lb. travel trailer, and in the long hot Texas summer the 165 T-stat does its part in keeping the engine a tad cooler. Now that the weather is beginnimng to cool off, I am going to replace it with a 180-deg. balanced unit, and see if that will drop the idle back to where it is supposed to be, but if it isn't I'm going to go back to the factory 195-deg. part, and consider the 165 experiment a very limited success that I won't repeat.
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