The optimum coolant combination is a 50/50 mixture of antifreeze and water. Any greater concentration does not significantly increase the efficiency of the coolant. Actually, the more antifreeze you add above 50 percent, the lower your protection becomes. The 50/50 mixture will produce freeze protection down to -34F (-36C), and boilover protection up 250F (121C).

Although regular water will do, the refilling of the system should be done with distilled water. Why bother with the extra expense? Distilled water doesn't contain any minerals which can dissolve and turn the coolant into a corrosive compound.

There are three ways (at least) to refill the radiator:

• Fill the radiator halfway with water. Can't see into the radiator? Estimate the amount of water based on the amount of coolant that you drained out. If you drained out two gallons (8 liters) of coolant, then you need to add one gallon (4 liters) each of water and antifreeze.
  
• Using a funnel, fill up the radiator with antifreeze.

A second way:

• In a separate clean container, mix antifreeze and water in a 50/50 ratio.
  
• Add the 50/50 mixture until the radiator is full, and store any extra coolant.

A third way:

• Pour in a half container of antifreeze. This usually comes in one-gallon (4 liter) containers, so add a half-gallon (2 liters).
  
• Add to the radiator an equal amount of a distilled water. Then mix water into the antifreeze container (which still has antifreeze in it). This will create the 50/50 mix, which you can add to the radiator until it's full.

It's good idea to have a little extra coolant after this fill-up, because the coolant level will drop slightly after you do Step 9. The excess from this step can be used to top off the radiator a little later.