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Quest for the Ideal Coolant Ratio

March 27, 2017

Our nation contains quite the variety of climates and seasonal changes. There are areas in Alaska that have seen a record -80°F, while Death Valley has seen a scorching 134°F record high. That’s a 214° swing!

While most of our cars may never be driven in this extreme weather, it’s safe to say that a good part of the country will experience anywhere from 15-100°F throughout a calendar year.  Contrary to popular belief, the water-to-anti-freeze coolant ratio is not a one-size-fits-all type of scenario, and in order to get the most out of our car’s cooling system, we’re going to want to play with some ratios.

There are four things to consider in regards to your coolant system: cooling, corrosion, boiling, and freezing.

Cooling: The most efficient way to transfer heat from the engine to the radiator is using pure water. In fact, most race cars run straight water (also because leaking anti-freeze is very slippery, and therefore dangerous on the track). But, keep reading.

Corrosion: The best water to use in a cooling system is distilled, as tap water contains calcium and magnesium—minerals that leave deposits over time. However, even a coolant system running only distilled water will still corrode over time.  Anti-freeze has engine protecting corrosion inhibitors. We need at least 10% anti-freeze in the cooling system to battle corrosion, even when using distilled water.

Boiling: Pure water has a boiling point of 212°F. At that temperature, bubbles prevent fluid-to-metal contact, and inhibit the transfer of heat from the cylinder head to the radiator. Pure anti-freeze, on the other hand, has a boiling point of 388°F. The problem with running pure anti-freeze is that the car won’t cool as well.

You may think that running a 50/50 ratio splits the difference and raises the cooling system’s boiling point to the middle—right at 300°F, right? Nope. At 90% anti-freeze, the boiling point actually drops to 284°F, and by 60% that figure tanks to 230°F. The decrease in anti-freeze from there on out has a more linear effect on the boiling point down to 212°F.

While raising the boiling point of our cooling system is good for when it reaches a higher temperature, we are better off not using it as a band-aid for a lack of cooling. Most cars are designed to run 175-205°F coolant temps, with the ideal number being right around the thermostat operation figure—about 185°F. If we’re able to keep the cooling system in that temperature range, we won’t need to worry about the boiling point as much.

Freezing: Anti-freeze does just as the name implies—prevents freezing in the cooling system. What’s interesting is that, when run by itself, it actually has a freezing point of around 10°F. However, when mixed with water at a 50/50 ratio, it can withstand temperatures as cold as -35°F before freezing.

In most places, including your local gas station, you can buy pre-mixed coolant, which is usually 50/50 anti-freeze and water. This standard ratio is an attempt to encompass as much of the nation as possible, and will provide great anti-freeze and anti-corrosion protection. However, this ratio isn’t optimal in warmer climates for cooling, as discussed earlier.

[The Optimal Coolant Ratio Depends on Your Climate]

So what is our optimal ratio you ask? To answer that, we simply have to figure out the coldest climate in which our cars will be driven in. Above we established that we need at least 10% anti-freeze for its anti-corrosive properties, so now we can use the chart below to help us find the ideal mixture.

% Water vs AF

Freezing Point

Boiling Point




















Since the lowest freezing point is achieved at a 40/60 water-to-anti-freeze ratio, there’s no added benefit to using more anti-freeze. Therefore, if you live in an area that will never see less than 26°F, your car’s performance will most likely be optimal at a 90/10 ratio year round.

If you live in an area that can see temperatures as low as -10°F in the winter, but can also get to 95°F in the summer, then you’re better off running a 60/40 ratio. However, if you’re one that may get stuck in traffic with the air conditioner on during the summer months, you may want to make the switch to a 90/10 ratio until winter hits again to get the most out of your car year round.

Make sure to read your vehicle’s owner’s manual to find the coolant system’s capacity so that you can figure out how much anti-freeze to add. Happy cooling!



[Additional Notes]

Generally, draining the cooling system can be accomplished by removing the drain plug at the bottom of the radiator. While this will drain the majority of the cooling system, keep in mind there will still be some coolant left in the cylinder block, which has its own drain plug in most vehicle.



After filling up the cooling system with the desired water-to-anti-freeze ratio, it’s important to remove any air bubbles from the system. To do this, turn the car on, and then turn the heater to full blast. Watch for air bubbles by loosening the screw. When the bubbles disappear and pure coolant oozes out, you’ve successfully “burped” the system.  For best results, have the front of the car elevated (for example, by jacking up the front of the car or simply being at the top of an elevated driveway), since bubbles always go up.

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