Cold is another reason 'your mileage may vary'

• Fuel use goes up as the temperature goes down
• Defrosters%2C heaters%2C thick fluids cut engine efficiency
• Use the garage%2C check the tires

In addition to all the other troubles it's caused this winter, cold weather is another reason 'your mileage may vary.'

Compared to fuel economy at 77 degrees, mileage declines 12% to 34% at 20 degrees. The shorter the trip, the bigger the drop.

And gas-electric hybrids are hurt the worst.

That, from the Oak Ridge National Laboratory. It compared the Environmental Protection Agency's official fuel-economy test results for 600 conventional vehicles and 14 hybrids under "normal" temperatures (around 77°F) and cold-weather conditions (20°F).

The lab found that regular winter driving in gasoline vehicles is about 12% less fuel-efficient at 20 degree than at 77.

In short-trip use -- three to four miles -- the drop can be 22%, Oak Ridge said.

That would slice a 20-mpg machine to 15 or 16 mpg.

Hybrids take the biggest hit. Their fuel economy drops 31% to 34%, the lab said. So a 45-mpg hybrid becomes a 30-mpg hybrid.

The reasons are pretty straightforward.

•The engine is slower to warm up to its most efficient operating temperature. In the three or four mile short trips it might never reach the proper temperature.

•Fluids are stiffer in cold weather, creating more friction that the engine has to overcome.

•Winter-blend gasoline can have slightly less energy per gallon than warm-weather blends.

•Tire pressure goes down, increasing the rolling resistance of the tires.

•Batteries don't work as well when it's cold. That means the engine has to work harder to spin the alternator to keep the batteries juiced-up, even if you use no extra electric accessories. It also means the regenerative braking on hybrids is less effective.

•Defroster, heated seats and other winter accessories require enough energy that they put drag on the engine as it tries to spin the alternator.

But, in plug-in hybrids and electric cars, using the seat heaters instead of the car's heater can extend the battery's range, the government says.

Government-suggested remedies likewise are common sense:

•Park in a warmer place (even if it means you have to clean out the garage to make room for the car).

•Keep the tires inflated to the proper pressure.

•Use the oil the automaker recommends. Old-timers will wince seeing that "5W20" recommendation on some modern machines, recalling the 10W30 or even 20W40 they used to use. But the lighter-weight (lower numbers) oil flows fast to key areas so the engine stays safely lubricated and warms up properly.

•If a lot of your winter driving is on the highway, wind resistance becomes a bigger villain because cold air is denser, harder to get through. So remove accessories that stick out and catch the wind, such as that roof-mounted luggage box.