[quote
Showing 200 on the gauge while on the highway is to be expected if you're running a 195 t-stat. Now if he was running a 180 t-stat and doing 200 on the highway, then I would say your cooling system does need some attention.

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Your thermostat only regulates the minimum temperature, not the maximum.

If an engine runs down the highway at 200 degrees with a 195 thermostat, then it will run down the highway at 200 degrees with a 160 thermostat. At best a lower rated thermostat will just delay the amount of time it takes to get to the system operating temperature.

The only time a thermostat will affect maximum temps (assuming no failure mode) is if the thermostat is artificially keeping the minimum system temperature above it's normal operating temperature.