In the distant past, the only reason that 160 degree thermostats were available was that they were for use with the old alcohol anti-freeze. Alcohol boils at about 170, and it would evaporate and be lost, leaving plain water behind. 180 degree stats were standard for plain water or glycol antifreeze. With a 7 pound pressure cap, a 190 is OK. True overheating occurs when the radiator can no longer dissipate the amount of heat that the coolant absorbs from the engine. The coolant temp then rises until it boils and lifts the pressure cap. One way to see the efficiency of a cooling system is to use a second temperature gauge to read the coolant temp at the lower hose connection. The difference in temperature between them is an indication of how well the radiator is doing its job.