Coolant/Water ?'s

4 quarts to a gallon, but I had trouble getting anywhere near 14.1 into my wife's 01. (I probably didn't get all the old out of it...)
I'd buy 2 gallons of Prestone and yes, you can use tap water.
Park on an incline with the nose of the GT pointed down the hill to drain it and be sure to remove the cap to allow air to back fill the gap left by the liquid leaving the system.
Then, to fill it, reverse the car. Nose pointed up the hill. (This is the most important part - if you don't have a good incline, jack it up or put it up on ramps to do this part) This will get the fill cap higher than any other portion of the cooling system and allow trapped air to escape - VERY IMPORTANT. Check the coolant level every day for the next week or so. And consider a bottle of Red Line's "Water Wetter" while at the Auto Parts store - some people swear by it. It is supposed to keep you up to 20 degrees cooler. Being in FL, I am going to try it on the next coolant chg.
Best of Luck!!!

FYI if you have too much water in the mix you will get a coolant dash light on. The sensor float will not float on water period, but floates perfectly on antifreeze (don't ask me why / how) but also if there is too much water in the mix the float will not float, and cause the light to come on. I had this problem recently trying to modify my mix. I ended up pulling my resivoir off and did that test on the sensor b/c of my damn coolant light. Look into some water weter too to lower coolant temps. Houston temps had me playing w/ my mix too.