Brain tease

[Phil Waterman]

With start of cold weather here in New Hampshire went to put antifreeze in my C60S which had only water and rust inhibiter in since engine swap this summer.

Here is the Brain Tease

Went to the manual looked up the capacity of the cooling system 13 quarts OK, so that means 6.5 quarts of antifreeze for a 50/50 mix which should give protection down to -34F OK. Drained off 2 gallons of water and added 7 quarts of antifreeze and topped of with some of the clean treated water that I had just drained. Took the truck out on the road to bring it up to temperature and be sure the antifreeze and water were well mixed. Ten mile drive around the lake and back to the house, then check the protection level of with antifreeze tester. Got a reading of +10 degrees Fahrenheit.

What happened?

I figured it out but what is your guess?