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.

  • get related content delivered to your inbox

  • manage my email subscriptions

ADVERTISEMENT

NY Yankees - LP: M. Pineda 0 FINAL
Toronto - WP: D. Hutchison 2
Detroit - LP: M. Scherzer 3 FINAL
Chicago WSox - WP: C. Sale 6
Chicago Cubs - WP: F. Doubront 5 FINAL
St. Louis - LP: J. Masterson 1
Cincinnati - LP: A. Simon 2 FINAL
Pittsburgh - WP: V. Worley 3
Minnesota - LP: J. Burton 2 FINAL
Baltimore - WP: A. Miller 3
Philadelphia - WP: J. Williams 7 FINAL
NY Mets - LP: B. Colon 2
Miami - WP: J. Cosart 4 FINAL
Atlanta - LP: A. Harang 0
Boston - LP: A. Webster 0 FINAL
Tampa Bay - WP: J. Odorizzi 7
Cleveland - WP: J. Tomlin 3 FINAL
Kansas City - LP: S. Downs 2
Detroit - WP: K. Ryan 8 FINAL
Chicago WSox - LP: C. Bassitt 4
Texas - LP: P. Klein 0 FINAL
Houston - WP: S. Feldman 2
Colorado - WP: T. Matzek 2 FINAL
Arizona - LP: V. Nuno 0
Chicago Cubs - LP: T. Wada 2 FINAL
St. Louis - WP: M. Gonzales 13
Los Angeles - LP: J. Wright 1 FINAL
San Diego - WP: T. Stauffer 2
Milwaukee - LP: M. Fiers 1 FINAL
San Francisco - WP: J. Peavy 3
Oakland - LP: J. Samardzija 0 FINAL
LA Angels - WP: Y. Herrera 2
Washington - WP: S. Strasburg 3 FINAL
Seattle - LP: R. Elias 1
Green Bay 9/4/14 7:30 PM
Seattle
BYU 35 FINAL
Connecticut 10
(6) Jacksonville St 7 FINAL
(8) Michigan State 45
Bowling Green 31 FINAL
Western Ky 59
TX-San Antonio 27 FINAL
Houston 7
Colorado State 31 FINAL
Colorado 17
UNLV 13 FINAL
Arizona 58
Penn State 26 FINAL
UCF 24
Troy 10 FINAL
UAB 48
(7) UCLA 28 FINAL
Virginia 20
Western Mich 34 FINAL
Purdue 43
(5) Ohio State 34 FINAL
Navy 17
Appalachian St 14 FINAL
Michigan 52
Ga Southern 23 FINAL
NC State 24
Boston College 30 FINAL
Massachusetts 7
West Virginia 23 FINAL
(2) Alabama 33
Rice 17 FINAL
(17) Notre Dame 48
(10) South Dakota St 18 FINAL
(24) Missouri 38
Fla Atlantic 7 FINAL
(22) Nebraska 55
California 31 FINAL
Northwestern 24
Marshall 42 FINAL
Miami-Ohio 27
UC Davis 0 FINAL
(11) Stanford 45
Arkansas 21 FINAL
(6) Auburn 45
(16) Clemson 21 FINAL
(12) Georgia 45
Liberty 29 FINAL
(23) North Carolina 56
Ohio U 17 FINAL
Kent State 14
Louisiana Tech 16 FINAL
(4) Oklahoma 48
Stephen F Austin 16 FINAL
(20) Kansas State 55
Fresno State 13 FINAL
(15) USC 52
Southern Miss 0 FINAL
Miss State 49
Texas-El Paso 31 FINAL
New Mexico 24
North Texas 7 FINAL
Texas 38
(1) Florida State 37 FINAL
Oklahoma State 31
(14) Wisconsin 24 FINAL
(13) LSU 28
Idaho 0 Postponed
Florida 0
South Dakota 13 FINAL
(3) Oregon 62
(25) Washington 17 FINAL
Hawaii 16
Utah State 6:00 PM
Tennessee
SMU 6:30 PM
(10) Baylor
Miami-Florida 9/1/14 7:00 PM
Louisville
Arizona 9/4/14 7:00 PM
TX-San Antonio
Pittsburgh 9/5/14 6:00 PM
Boston College
Washington St 9/5/14 9:30 PM
Nevada
SMU 9/6/14 11:00 AM
North Texas
Fla Atlantic 9/6/14 11:00 AM
(2) Alabama
Buffalo 9/6/14 11:00 AM
Army
(24) Missouri 9/6/14 11:00 AM
Toledo
Arkansas State 9/6/14 11:00 AM
Tennessee
(8) McNeese State 9/6/14 11:00 AM
(22) Nebraska
Central Mich 9/6/14 11:00 AM
Purdue
Akron 9/6/14 11:00 AM
Penn State
Western Ill 9/6/14 11:00 AM
(14) Wisconsin
(20) Kansas State 9/6/14 11:00 AM
Iowa State
(4) Oklahoma 9/6/14 11:00 AM
Tulsa
Western Ky 9/6/14 11:00 AM
Illinois
SC State 9/6/14 11:30 AM
(16) Clemson
Navy 9/6/14 12:00 PM
Temple
UAB 9/6/14 1:00 PM
Miss State
South Alabama 9/6/14 1:00 PM
Kent State
New Mexico St 9/6/14 1:00 PM
Georgia State
Colorado 9/6/14 2:00 PM
Massachusetts
Fresno State 9/6/14 2:00 PM
Utah
(1) Eastern Wash 9/6/14 2:00 PM
(25) Washington
(15) USC 9/6/14 2:30 PM
(11) Stanford
Ohio U 9/6/14 2:30 PM
Kentucky
Middle Tennessee 9/6/14 2:30 PM
Minnesota
Northern Ill 9/6/14 2:30 PM
Northwestern
Missouri State 9/6/14 2:30 PM
Oklahoma State
Ball State 9/6/14 2:30 PM
Iowa
Maryland 9/6/14 2:30 PM
So Florida
Eastern Mich 9/6/14 3:00 PM
Florida
Georgia Tech 9/6/14 3:00 PM
Tulane
(18) Ole Miss 9/6/14 3:30 PM
Vanderbilt
Old Dominion 9/6/14 5:00 PM
NC State
(8) Michigan State 9/6/14 5:30 PM
(3) Oregon
(19) Arizona State 9/6/14 6:00 PM
New Mexico
Louisiana Tech 9/6/14 6:00 PM
Louisiana
San Jose St 9/6/14 6:00 PM
(6) Auburn
East Carolina 9/6/14 6:00 PM
(9) So Carolina
Duke 9/6/14 6:00 PM
Troy
Idaho 9/6/14 6:00 PM
ULM
(17) Sam Houston St 9/6/14 6:30 PM
(13) LSU
Michigan 9/6/14 6:30 PM
(17) Notre Dame
Lamar 9/6/14 6:30 PM
(21) Texas A&M
Northwestern St 9/6/14 6:30 PM
(10) Baylor
BYU 9/6/14 6:30 PM
Texas
Citadel 9/6/14 6:30 PM
(1) Florida State
San Diego St 9/6/14 7:00 PM
(23) North Carolina
Virginia Tech 9/6/14 7:00 PM
(5) Ohio State
Memphis 9/6/14 9:00 PM
(7) UCLA
Colorado State 9/6/14 9:15 PM
Boise State
Air Force 9/6/14 9:15 PM
Wyoming
Oregon State 9/6/14 9:30 PM
Hawaii
Texas Tech 9/6/14 10:00 PM
Texas-El Paso
Colorado 0 FINAL
Seattle 1
New England 3 FINAL
Toronto FC 0
Columbus 0 FINAL
Montreal 2
FC Dallas 0 FINAL
Chicago 1
Real Salt Lake 1 FINAL
San Jose 1
Portland 3 FINAL
Vancouver FC 0
Ottawa 10 FINAL
Montreal 20
Winnipeg 3:00 PM
Saskatchewan
Chicago 70 FINAL
Indiana 77
Connect with twitterConnect with facebookConnect with Google+Connect with PinterestConnect with PinterestConnect with RssfeedConnect with email newsletters

ADVERTISEMENT

ADVERTISEMENT

ADVERTISEMENT

 
Close