I the winter, my 96 with the stock thermostat would loose 3-4 mpg. If the cylinder temp is to low you will loose efficiently. However I was told by Cummins 180 degrees is optimum. The 12 valve would hang around 160 thats why the mpgs were low. So 180 degree thermostat should be fine, but running at 200 degrees may give us less smoke, better combustion & more mpgs.
Running the 180 could benifit the auto tranny temps witch might be more important. What do you guys think????, doug