Do all cars get lower gas mileage during the winter months?

Dear Car Talk

Dear Car Talk | Mar 01, 1997

Dear Tom and Ray:

I have a 1996 Honda Civic. From June to October, it got 34 miles per gallon. From mid-October through February, it got only 25 mpg. The dealer says "all cars get lower mileage in the winter due to the reformulation of gasolines." They refuse to look at it without charging me. Whatever happened to customer service? Cripes, the car cost almost $18,000, and it seems to me they ought to be willing to give it a quick lookover in the first year. Do you think there's something wrong with my car?--John

TOM: Maybe, maybe not, John. Mileage does drop in the winter, but it has nothing to do with the reformulation of gasoline. It has to do with the reformulation of the temperature, from nice and warm to freezing cold.

RAY: Several things conspire to lower your mileage in cold weather. One is lower tire pressure. Even without any leaks, tire pressure drops about a pound for every 10-degree drop in temperature. So if you haven't checked the pressure since it was 80 degrees in the shade last summer, you may have lost enough tire pressure to seriously reduce your mileage.

TOM: The other important change that occurs in the winter is that gasoline doesn't burn as well in cold temperatures. So you're not burning all of your gasoline completely. That means that some of it is being wasted in the cylinders, and that's hurting your mileage.

RAY: This is particularly true if you make short trips, because the engine gets out of the "warm-up" mode, where it boosts the idle speed and is pouring in extra gasoline anyway.

TOM: Plus, when it's really cold, most people DO make more short trips. Why? Because it's cold! You might normally walk three blocks to the store to get a quart of milk. But when it's 2 degrees out, are you going to walk? No! You'd probably drive so you don't freeze your little tootsies off.

RAY: So some degradation in mileage in the winter is considered normal. But your gas mileage has dropped 25 percent, and that's a lot. So if I were your dealer, I'd at least check to see if your thermostat is within specifications. And you're absolutely right that this should be done -- cheerfully -- under warranty.

TOM: If the thermostat is out of spec, and is keeping the engine from warming up all the way, the computer would be fooled into thinking that the car is always in warm-up mode, and that could explain the extra gas you're using.

RAY: But if the thermostat checks out fine, the tire pressure is correct, and an emissions test shows nothing else obviously wrong, then I'd wait for the warmer weather to return to your area and see if your mileage goes back up to 34. It might, in which case you've just got an exaggerated case of normal winter mileage syndrome.

But if it doesn't return to normal, then bring your car back to the dealer and insist that they figure out what's wrong with it. And if they keep treating you like a halitosis poster boy, contact the Honda zone manager and complain.

TOM: Or come back with several large friends -- that's what our customers do.

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