I have just replaced my thermostat but i was unable to get a 190 degree thermostat which is what my bronco had stock. Instead I had to use a 180 degree thermostat. I'm in South FL so the summers are pretty hot and I tow frequently so I was under the impression that cooler was better but a friend told me I would have dramatically reduced MPG, is this true? Are there any negitive side effects of running a cooler thermostat?
One factor is the thermostat, a hotter engine gets better fuel economy while a cooler one can develop more power. However, don't forget we are not talking about the most economical vehicle in the world here. Since you are putting the additional stress on the engine of towing, the important thing is to look at the overall cooling system. I'll bet you even want to run A/C while doing that

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[email protected] 2x" width="20" height="20" /> . Then more important than your tstat is the radiator size and something like a transmission oil cooler.
As far as running a cooler thermostat for hot weather, if your engine is overheating there is some problem with your cooling system. I run a 195 tstat all year long and never have a problem. Some will argue that it is because it doesn't get that hot on the East coast, BUNK. I run plenty on 4 lane highways that have air and surface temperatures equal to any desert conditions. The key to keeping the engine from overheating, is a cooling system that is efficient and large enough for the environment it is in. My engine runs rock solid at 195 year around no matter what the air temperature is. You might notice from my signature though, that I don't have a stock fan, I also have a transmission cooler and a over sized radiator.
People who run stock cooling systems at low RPM such as off roading in the deasert, for example, have some strikes against them. The main one being that their water pump is running at a very inefficient speed. One of the solutions there is to run an electric pump that keeps a high volume of water moving thru the system. Another solution is to run a larger than stock radiator to increase the cooling capacity plus a high volume electric fan.
Oh well, enough babble.

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