Why does gas mileage drop in winter?

  • Article by: PAUL BRAND , Star Tribune
  • Updated: December 29, 2005 - 4:53 PM

Q I am wondering why my '98 Ford Ranger gets noticeably poorer gas mileage when the weather gets colder. The vehicle has 110,000 miles on a 4.0L engine. It is four-wheel-drive. During the summer months I can get 16 to 18 mpg. In the winter it drops to 14 to 15 mpg. I keep the tires properly inflated and let the car idle very infrequently, other than to warm it up for a few minutes. Does the cold air somehow affect gas mileage? Does operating the heater affect gas mileage, much like the AC does in the summer? I notice a drop in mileage even when I don't use the 4WD.

Q I am wondering why my '98 Ford Ranger gets noticeably poorer gas mileage when the weather gets colder. The vehicle has 110,000 miles on a 4.0L engine. It is four-wheel-drive. During the summer months I can get 16 to 18 mpg. In the winter it drops to 14 to 15 mpg. I keep the tires properly inflated and let the car idle very infrequently, other than to warm it up for a few minutes. Does the cold air somehow affect gas mileage? Does operating the heater affect gas mileage, much like the AC does in the summer? I notice a drop in mileage even when I don't use the 4WD.

A Three words: "time to temperature." In cold weather, our vehicles take a much longer period of time to reach full operating temperature. And they take this extra time each and every time we start them up, even if they have not fully cooled down.

Modern engine-management systems are very efficient at optimizing the fuel/air ratio entering the engine. The oxygen sensor monitors the percentage of oxygen in the exhaust, compares this with the percentage of oxygen in the atmosphere, and generates a low-voltage signal that communicates this ratio to the computer. The computer then adjusts the pulse-width of the fuel injectors -- the precise period of time each injector is open on each injection cycle --to fine-tune the amount of fuel reaching each cylinder.

The system makes this very fine adjustment dozens of times each second, working very hard to reach the optimum air/fuel ratio for any given situation, and at steady-state cruise speed seeks to approach the perfect ratio of 14.7 to 1, called the stoichometric ratio. The system is running in a "closed loop" when it is relying upon the oxygen-sensor signal to fine-tune engine operation.

But the engine-management system can seek this optimum air/fuel ratio only when the engine is up to full temperature. In fact, the computer does not look for a signal from the oxygen sensor until it approaches full temperature. During the warm-up cycle -- which takes considerably longer in cold weather -- the computer operates on a warm-up program based on coolant temperature, mass airflow or MAP sensor input of air volume and temperature, throttle position and engine rpm. This is called "open loop" operation, meaning the system is not operating off the feedback from the oxygen sensor.

In open-loop operation during the warm-up period, the engine requires -- and is provided with -- a richer air/fuel ratio to ensure good combustion.

It needs this extra fuel for the simple reason that a percentage of the atomized, then vaporized, fuel delivered to the engine condenses into liquid gasoline on cold internal engine components such as the intake manifold, intake valves, pistons and cylinder. And remember, it's gasoline vapor, not liquid, that burns.

Until those parts warm up, the engine needs more fuel to operate with reasonable drivability. Thus, the colder the weather, the longer it takes for your engine to reach full operating temperature and closed-loop operation, and thus the more fuel the engine uses to deliver the same driving cycle. That's the primary reason your engine consumes more fuel in winter driving.

Another factor in winter mileage is air density, defined as the number of air molecules per cubic foot of air entering the engine. Remember that 14.7-1 air/fuel ratio? That means 14.7 parts of air are mixed with one part of fuel for the perfect air/fuel ratio -- that's a lot of air!

In winter, colder air means denser air -- more molecules per cubic foot. At any specific throttle setting or opening, the same number of cubic feet of air, but containing more air molecules, will enter the engine. The computer will provide more fuel to create the proper air/fuel ratio at that moment. This is a bit more subtle, since "more air/more fuel" produces more power, so you may be able to operate at a slightly lower throttle setting -- sort of rebalancing the equation.

The bottom line with your Ranger is that you are experiencing a normal reduction of fuel mileage -- about 10 percent -- in winter driving. Be thankful it's not more than that!

Q Recently I saw a brochure promoting the use of dry nitrogen for tires in place of air. It claims to maintain tire pressure better, keep tires cooler, use less fuel, prolong the life of the tires, reduce tire failures by as much as 80 percent, and improve handling and performance. Please give us your thoughts on this.

A I first learned about the advantages of nitrogen over plain old air in racing tires. One of my jobs when working for the factory Jaguar race team back in the 1970s was to visit the local welding supply shop near each race track we ran on and rent a tank of nitrogen for the weekend.

The primary advantage of nitrogen in tires used for racing is that tire pressures change much less with changes in temperature. As the race tire slides across the pavement, it heats up, causing an increase in pressure. Nitrogen-filled tires don't build pressure as much, thus tire pressures are better controlled for optimum performance.

That same advantage works for street tires as well, particularly this time of year. Tires filled with nitrogen won't decrease in pressure as much as air-filled ones when the outside air temperature drops. Remember, air-filled tires drop roughly one pound per square inch for each 10-degree drop in temperature.

Secondly, nitrogen is moisture-free, meaning less condensation and corrosion inside the tire -- including the wheel itself. Thirdly, nitrogen tends to be less prone to leakage through porosity of tires and wheels.

The additional claims you mention are spin-offs of the three basic advantages. "Cooler, less fuel, longer life, fewer failures and improved handling" all revolve around the reduction in pressure loss over time with nitrogen. If motorists kept their tires properly inflated with air, those issues would be far less significant.

The bottom line? Nitrogen is a good thing for your tires. And better tire shops are now offering and using nitrogen to inflate tires.

Happy holidays, buckle up, and have a safe New Year.

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