My owner's manual says to use half antifreeze and half water, but my mechanic says just add antifreeze.

Dear Car Talk

Dear Car Talk | Oct 01, 1994

Dear Tom and Ray:

Last fall, when I had my radiator flushed and coolant changed on my 1989 Ford Escort, my mechanic said it would be fine to put all antifreeze in the system. My owner's manual says to use half antifreeze and half water. Who is right?

TOM: Your owner's manual is right, Joe. You should always use a 50/50 mixture.

RAY: There are three things you need your coolant mixture to do. You want it to keep the engine from freezing in the winter; you want it to absorb heat to keep the engine from overheating; and you want it to keep the cooling passages inside the engine from rusting.

TOM: Straight antifreeze freezes at about zero degrees Fahrenheit. But when you mix it with 50% water, the freezing point DROPS to about 40 below zero! So your freezing protection is actually better with a 50/50 mixture.

RAY: And believe it or not, you also get better heat absorption in hot weather with a 50/50 mixture, so the mixture is more likely to keep your car from overheating. Although water is also a pretty good heat absorber by itself, it's boiling point is not as high as the 50/50 mixture.

TOM: The only contest pure antifreeze wins is in the rust department. Antifreezes contain rust inhibitors, and straight antifreeze would probably give you the best rust protection. But research shows that as long as you have 50% antifreeze in the mixture, the rust inhibitors will do their job just fine.

RAY: So unless you live in a part of the country where polar bears have a majority on the city council (in which case you can use a 70/30 mixture of antifreeze to water), 50/50 is the optimal mixture to balance freezing point, heat absorption, boiling point, and rust protection. Plus, if you use a 50/50 mixture of antifreeze and water, you'll cut your annual antifreeze bill in half. And since you have an '89 Escort, you can start saving up for the more pressing engine repairs that are certain to befall you.

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