Losing a gallon of antifreeze every 50 miles

Never add just water, you can use 50/50 ,or mix 1 gallon regular antifreeze with a gallon of distilled water to get 2 gals of 50/50.

You should look under the car after it sits a while and see if there is antifreeze on the ground. If so try to see where it could be leaking under the hood directly over the puddle. You could have a leaking water pump. If so there would be dampness under the right side of engine compartment. Also check the radiator hoses at front of car.

If you've added two gallons of antifreeze, you need to find out where it's going. Make sure the antifreeze you are adding is diluted with water in a 50/50 solution. Pure antifreeze has little freeze protection and boils quickly. If there is no visible leak, check your oil for coolant contamination. Fill the radiator and leave the radiator cap off. Put the heater controls on hottest setting, but do not turn the heater on. Start the engine and watch the coolant. If the coolant level drops, add more to full. If the coolant is flowing thru the radiator when you first start the engine, the thermostat is stuck open and needs to be replaced. If the water is not flowing, then keep an eye on it and wait for the thermostat to open. Add antifreeze to keep the radiator full. Look for air bubbles or lots of foaming in the coolant. That would indicate a blown head gasket and the reason you are losing antifreeze. If there's no bubbles, and the coolant level stays full, put the radiator cap back on and see if the heater is working better.

sytem capacity is 8.6 qts. you want a 50/50 solution, so drain alittle more than a gallon of the straight water out, and add 4.3 qts. of dexcool antifreeze or equivalent. i always go slightly stronger just to be sure, but not too much antifreeze as it doesn't work straight. no more than 4.5 qts.

The dealer recommend GM Dex Cool Antifreeze.
But there are complaint against this type of coolant.
http://www.stevesnovasite.com/forums/archive/index.php/t-128547.html
In GM cars we replace Dex Cool with Prestone green coolant.
It's up to you. Ask before but if you decide to change the type of coolant then:
If your intention is to add coolant only, then you need:
Mix with
ANY COLOR ANTIFREEZE
ALL MAKES ALL MODELS
of cars and light trucks
add to ANY color antifreeze
PRESTONE
50/50
PREDILUTED ANTIFREEZE/COOLANT
EXTENDED LIFE up to 150000/mile 5 year protection.
DO NOT ADD WATER.

If your intention is to flush the RADIATOR (Cooling System), then you need:
Mix with
ANY COLOR ANTIFREEZE
ALL MAKES ALL MODELS
of cars and light trucks
add to ANY color antifrezze
PRESTONE
EXTENDED LIFE up to 150000/mile 5 year protection.
On the back you have to read directions:
Step 1 Drain. Step 2 Flush. Step 3 Fill.
Recommended to ad 30% to 50% volume, distilled water. If you flush the system and rinse with water you must add only 30% water to the antifreeze. There is more water trapped inside the engine compartment, because the thermostat is closed. Make sure to have a coolant density tester. When you finish one of 5 balls must sink, that indicates that you reached 50/50 strength. That means 50% Prestone and 50% Water, which will provide to your car a -34F degree FREEZE-UP Protection , and +265F degree BOILOVER protection.The next day,with a cooled engine open radiator cap, and test the strength of coolant. If not quite right, then siphon coolant from the radiator and add antifreeze, or distilled water as necessary.
Make sure to disposal the used coolant properly.

If you are draining your radiator, or just need to add some, there are a couple of options. You can either buy antifreeze in a 50/50 premixed solution, or purchase one gallon of antifreeze and 1 gallon of distilled water (less than $1.00 at your local grocery store) and mix equaly in another container that way you have a proper mixture in your radiator. The 50/50 mixture comes in an orange color and the full strength comes in the traditional green color.

If you have no visible signs of oil leaking out of the engine then the oil is being burnt. By visible leaking I mean a small puddle of oil under the car after having been left overnight. This can be cured by replacing shaft seals and gaskets. Burning oil is visible as smoke under deceleration (no throttle) and under full power(flooring it). This is due to worn/old valve stem seals(smoke under deceleration) and worn oil rings(smoke under acceleration) This can only be cured by a major engine overhaul or engine replacement. Bear in mind that losing a pint of oil in 1000 miles is acceptable.

2 gallons and 2 galons water should do it

loosing this much it is leaving puddles under it? possible causes would be water pump. hoses, intake gasket. what area do you see a puddle? check oil and see if it is milky