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It's Winter ... What's Happened to my Car's Fuel Economy?

Cold Weather Kills Fuel Mileage

By

2008 Nissan Altima Hybrid in the snow - hybrid in the winter weather

2008 Nissan Altima Hybrid in winter snow showers.

photo © Scott Gable

Numb fingertips. Icy toes. Frosty windshields and frozen locks. Just when the cold blustery days might be getting you down, you notice that it’s taking more fuel to fill-up your vehicle. While it may be tempting to think that your calculator's batteries just need replacing, it’s time to face the cold, hard truth:

Winter conditions do cause a drop in fuel economy.

But wait—don’t make plans to move south yet. Grab your laptop and snuggle up by the fire or woodstove and take five for a few quick tips to keep more bucks in your wallet.

Old Man Winter’s Pinch

So, why exactly does colder weather throw a wrench at most vehicles’ regular fuel economy? Well, there are lots of reasons actually, but friction is a big one. While it's always an issue, cold weather increases friction on almost every front. Most vehicle fluids, from motor oil and transmission fluid to power steering fluid and differential gear oil, are thicker and more resistant to flowing when cold. Translation: Harder to pump, more work for the engine. Another less obvious source of friction is rolling resistance.

First of all, cold temperatures lower the pressure in your vehicle’s tires by 1 to 2 PSI for every 10 degrees that the temperature plummets. Secondly, when the weather turns foul, vehicles must work harder to push through snow and slush. Both of these conditions equal greater rolling resistance and more fuel used to cover the same distance. Add to that the fact that gasoline doesn’t atomize and burn as well in cold temperatures; trace amounts of unburned fuel are left in the cylinder and evacuated with the exhaust. Yes, it’s a double-whammy. Not only does that unburned fuel equal lost power, it also substantially increases your vehicle’s emissions. But that’s not all—throw in heavy use of the heater, winter gas formulations and short trips to and fro, and it’s a recipe for pain at the pump.

What’s Happening under that Hood

All vehicles have an optimum operating temperature, and the colder it is, the longer it takes to get there. While modern engines make very efficient use of the fuel mixture entering the engine, they rely upon the oxygen sensor to monitor the O2 content of the exhaust, as well as sensors for manifold pressure, mass air flow, throttle position, and coolant temperature, among others. All of this information is communicated to the vehicle’s computer that continually adjusts ignition and valve timing as well as fuel injector pulse width, to fine tune the amount of fuel delivered to each cylinder. This is done many times per second. But here’s the caveat—this engine-management system is only at tip-top efficiency when the engine is at full operating temperature. And when the mercury’s showing low, all vehicles require longer periods of time to reach that full operating temperature—and it’s in that warm-up period that efficiency is lost.

Add to that the common practice of making short trips (to minimize running to and fro in sub-zero temps and wind chills), and each time your vehicle cools down it has to come back up to optimal operating temperature. It all takes its toll.

Bottom Line: Here are the Tips

That’s right. We’re not here to bring on an even worse case of SAD (Seasonal Adjustment Disorder). We believe in being proactive and taking the bull by the horns … well, you get the idea. Anyway, here are some quick and easy ways to minimize the dip that your vehicle’s fuel mileage takes in the winter months:

  • Check your tire air pressure monthly—and keep it in the recommended range. Even if you don’t have an air compressor, you can invest in a tire-checking gauge and then swing in to the gas station. That’s five minutes that can save more than $5/month.

  • Plan your trips ahead of time—and do all your errands in one area at one time. By minimizing the stop-and-start, sporadic on-and-off operation of your vehicle in sub-zero temperatures, you’ll not only save time, you’ll minimize the amount of time your vehicle spends in warm-up mode.

  • Go easy on the accelerator. We can all take this advice. And it’s easily worth 1-2 mpg. Avoid jack rabbit starts and heavy motor-down mama driving. The guy in front of you will appreciate it—and the results will show up on your next fuel bill; plus, it might help prevent a wipe-out on black ice.

  • When possible, park your vehicle in a garage to keep it warmer, thereby shortening the time needed to reach full operating temperature.

  • Consider a block heater, an oil pan heating pad or a fuel heater for cars parked outside in frigid parts of the country; the initial expense will be recouped many times in added fuel efficiency.

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