Flushed radiator how much coolant do I need

The amount is listed in the service manual, but we don't normally pay attention to those numbers. The reason is there is going to be a lot of water left in the engine block and heater core that doesn't drain out. If you mix new antifreeze with water at a 50/50 mixture, as recommended, you'll end up with not enough antifreeze and a freeze point that isn't low enough.

Instead, use compressed air if necessary to empty the reservoir, then start refilling the radiator with a gallon of straight antifreeze. Next, add a half gallon of water, then a half gallon of antifreeze. If you can't get that all in, finish filling in the reservoir. Once the radiator is full, drive the vehicle through one or two warm-up cycles to burp the air and mix the fluids. If there's a bleeder screw on the thermostat housing, open that while you're filling the radiator. If there isn't one, pay close attention that the engine doesn't overheat. If it starts to, we'll have to discuss removing a plug or sensor to burp the air pocket under the thermostat.

Once the coolant is mixed, check the freeze point. Add antifreeze or water as necessary to the reservoir to get the freeze point to around minus 35 to minus 45 degrees. Lower is not better because the freeze point will actually start to go up. The temperature indicated on the tester will be inaccurate because it goes by weight of the mixture, not chemistry. Also, you need enough water because it carries more BTUs of heat to the radiator than antifreeze does, and you need enough antifreeze to get the needed additives in it.