This is a typical BITOG OCD type of question that only we ask ourselves. My infiniti in the signature, I had to do an emergency thermostat replacement on it in april . Installed a OEM nissan thermostat and gasket, i didn't have time to get a coolant. I just went to walmart and got a gallon of distilled water. Used almost the fill gallon and everything is all said and done. Fast forward to today, I just thought about my coolant/water mixture and the upcoming winter. Being in chicago where we can see -20F sometimes. My FSM states i have 9 1/4 US quart of coolant, that is about 2.3gal. My simple math shows that my current system has about 30/70 coolant/water mixture (1.3g of 50/50 mix and 1g of water) Think i'm good for the winter? My car doesn't sit outside much, inside my garage or in my heated work garage. Should i just do a drain and fill with Zerex asian which is a 50/50 mix, to bring it up to near 40/60. Or do something else... what temperture is a 30%coolant mixture good for? And is my math right?