Slow,

The "temperature" of a thermostat is the temp at which the 'stat opens. It controls how fast the engine warms up.

My understanding is that you want the engine to run in the 180-220 range for best efficiency. The 165 or 180 thermostat might be too cold unless you have an old cooling system or are running a lot of engine -- at 180 degrees the system would be working at full power to reduce engine temps. Of course, if you're running in slow traffic or very hot weather, this might compensate.

What I usually do is put one in and run it for a while to see how I like it -- they're easy to change.

Brian

My 1960 Chevy 1/2-Ton !