When adding antifreeze to a car, owners should stick with the manufacturer's recommendation, according to ChiltonDIY. Manufacturers include their recommendation in the operator's manual. Since tap water contains minerals, owners should use distilled water to dilute the antifreeze to the optimal 50 percent mix.
Until the early 1980s, vehicles used glycol-based antifreeze. Glycol antifreeze requires flushing the radiator and replacing the coolant mix at least every three years. The newer mixes offer longer life. Mixing does not increase the corrosion resistance of modern antifreeze mixes, as of April 2015. However, adding inferior antifreeze shortens the lifespan of the coolant in the radiator.