Why do I get worse gas mileage in the winter?

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  1. edge

    edge Well-Known Member

    I thought the idea is the colder the air is The denser the oxygen, the better fuel/air ratio. Is it the ethanol added in the winter that is killing my mileage?

  2. Gmash

    Gmash Well-Known Member

    Cold air makes the oil thicker, takes the engine longer to get to optimal operating temperature, time idling waiting for the car to warm up, just generally makes the engine work harder until it warms up.
  3. Speed Daemon

    Speed Daemon Disabled

    Actually the kind of motor oil that's used here in the US (e.g. 5W20) is thinner at low temperatures.

    I wouldn't count time spent "warming up" a car as a contributor to gas mileage because the car isn't moving toward a destination at those times. This practice (along with running the car with the A/C on to cool down the interior) does waste fuel, and isn't necessary with modern cars.

    Things that can cause increased fuel consumption while the car is being driven includes road conditions. When roads are wet, or made slippery by ice, snow and even residual matter like salt and sand, cars can lose traction and burn up fuel spinning their wheels. For cars with electronic stability controls, the fuel is lost to heat when the ESC applies brake pressure to wheels to stop them from slipping. And of course using AWD or 4WD systems uses more energy.

    Another thing to consider is that winter air tends to be colder, which makes it more dense. Modern fuel injection systems see that density, and add more fuel to maintain the right air/fuel ratio. That means that a car can produce more horsepower in the winter than in the summer. Those who don't adjust their driving habits to slow down in bad weather, or those who must apply full power will burn more fuel because of that.

    Ethanol is used all year where I live. The gasoline itself is reformulated in the summer months to produce less smog in some areas. I don't know if one formulation is significantly less efficient, but it's a possibility.

    Ethanol has a lot less power density than pure gasoline, so miles per gallon will drop as ethanol content goes up. E85 users will see the worst miles per gallon because it's 85% ethanol.

    I think it's a combination of all of the above and more.
    johnlgalt likes this.
  4. javasirc

    javasirc Well-Known Member

    The primary reason gas mileage is less in the winter is because the air is denser. It takes more energy to push denser air out of the way. The effects can be lessened by driving a few miles per hour slower.
  5. junebug1701

    junebug1701 Member

    If you live up North, the oxygenated gasoline used in the winter will lower your MPG. In January of '09 I drove from S. Louisiana on up to Canada and the mileage dropped the farther North I traveled. We don't have winter/summer gas formulations down here so we don't normally see it.
  6. mplevy

    mplevy Well-Known Member

    The computer runs the engine richer to get to operating temperature faster.

    Yes speed daemon, warming (or cooling) a vehicle without moving WILL contribute to lower MPG. You're burning fuel and not accumulating miles, effectively getting ZERO MPG during that time.

    Last week we had negative temps, I didn't significantly change my startup procedure (let it idle the same as normal or a little longer), and I got 2 or 3 MPG less.
  7. Speed Daemon

    Speed Daemon Disabled

    I think that you may have missed my point. Although at first glance it appears that you're getting 0 MPG, and the crude MPG meters in some cars will report it as such, the fact is that in a static condition like this, distance (miles) is a constant, not a variable. That means that "M" is always zero while the vehicle is parked. To reduce it further, there is no M in static situations.

    If you have a constant zero as a numerator, the result is always the static number zero, which makes "measuring" MPG pointless under static conditions. Nobody puts a MPG meter on stationary motors for that very reason. In other parts of the world where the formulas use distance as the denominator (e.g. "liters per kilometer"), having a denominator of zero gives no quotient at all!

    Under dynamic driving conditions, it takes more sophisticated math (like calculus) to factor in the necessary start/stop cycles in driving in traffic. The complex math can be reduced to a single scalar number that we call MPG. But there's a lot more to it than that. :)

    In short, fuel consumption is not the same as gas mileage.

    Today we hit a low for the season in Madison. When I went out to pick up a package, my gas mileage was reduced by slippage due to the ice and snow covered pavement, parasitic losses due to the use of 4WD, and an especially long time it took to park in my indoor garage as my windows fogged over. I thought it was interesting that I had to take as long to park as the rest of the trip! But that's an excellent example of how there are many contributing factors to this complex equation. :)
  8. nickdalzell

    nickdalzell Well-Known Member Contributor

    ever since i dumped the dinosaur that was my 1984 Fifth Avenue for a more modern 1992 Bonneville SSE, my gas mileage has stayed the same in all four seasons. you only notice a drop if you got a old carburetor car that is very cold natured, which dumps gas into it at an alarming rate when cold (as evidenced by the black smoke and carbon stains on snow first thing in the morning).

    last car i had with a carb was my AMC AMX, and it was slightly better but still noticeably lower in winter. older cars don't take too kindly to corn ethanol either.

    1984 Chrysler Fifth Avenue, 318 V8, 3-speed auto: 9 mpg highway winter, 13-18 highway summer
    1980 AMC Spirit/AMX, 258 straight six, 4-speed manual: 18 mpg hwy winter, 20-21 hwy summer
    1992 Pontiac Bonneville SSE, 3.8 V6, 4-speed auto: 29-30 mpg hwy winter, 33-34 mpg hwy summer
  9. Gmash

    Gmash Well-Known Member

    While miles per gallon and fuel consumption may be two different things in math class, in the real world I think most people would consider them synonymous.
    Nick, 3-4 MPG is a pretty significant drop in mileage.
  10. Speed Daemon

    Speed Daemon Disabled

    The technology for improving cold start performance in automotive engines has improved considerably over the years. Manual choke valves were the least efficient, as you were just guessing how to set them, and could forget to disengage them after the motor was warm. Carbs with bimetal thermostatic controls worked better, but were still crude. Electronic fuel injection was a quantum leap forward, since it could monitor various parameters for best efficiency. And direct injection has all but eliminated the need to enrich the fuel mixture.
  11. pbf98

    pbf98 Well-Known Member Contributor

    Another contributor that I have noticed myself would be those "over" cautious drivers that will drive one speed then slow down, and then speed up again. Varying your speed does bring down your gas mileage. It is best to try and go a speed you are comfortable driving with in the current conditions and stick close to it.
  12. nickdalzell

    nickdalzell Well-Known Member Contributor

    you would also be amazed how tire health changes things too. tire pressures drop in colder weather.
  13. nickdalzell

    nickdalzell Well-Known Member Contributor

    i am so glad i no longer have to listen to my Fifth Avenue chugging a rough carbon-producing idle for the ten minutes it took to warm up, made worse by the stumbling ride it gave me had i tried to shift to drive right after i started it up. as i see it carburetors can die a slow, agonizing death for all i care. i swear the gas needle would drop almost 1/8th the gauge if i let it chug away doing nothing
  14. mplevy

    mplevy Well-Known Member

    While EFI may have helped to improve fuel economy, people wanting heat - and wanting it YESTERDAY - as well as the desire to get the engine to optimal running temperature for peak efficiency, calls for the engineers to enrich the mixture to speed up warm up times.

    Now, my 4 cyl. Subaru (2011) doesn't warm up as fast as my 6 cyl. Pontiac (2006) did but the Subie gets considerably better MPG despite the AWD, brick-like aerodynamics and heavier vehicle. Yes, it's a 4 cyl. but it's also less powerful for a heavier vehicle with more drag. Yes, I see a significant drop in MPG in the winter, especially when the temps go below 20, but my avg. MPG for a tank never comes close to matching the temperature like it did on the Pontiac sometimes.
  15. Speed Daemon

    Speed Daemon Disabled

    Ah...the last time I checked, it was the responsibility of the vehicle's operator to make sure that tire pressure is correct regardless of season.

    My Mustang has UHP summer tires that are sensitive to differences in inflation as little as 1 PSI. At 33 PSI exactly, the tires hold like glue. At 32 PSI they turn into smoke machines when trying to accelerate out of turns.

    My 4x4 has tires rated at 32 PSI, which is what I keep it at in good weather. But I bump that up to as much as 35 PSI in order to reduce the contact patch and keep the tires planted better. It also gives me a margin of safety for when temperatures drop. IME a 10
    johnlgalt likes this.
  16. Speed Daemon

    Speed Daemon Disabled

    Actually richer mixtures create excessive endothermic cooling when the fuel evaporates, which slows the engine's ability to heat the passenger compartment.

    The only reason for enriching the mixture in the first place is because fuel atomization is inhibited at lower temperatures. That's why I took note of how technology, especially direct injection solves the atomization problem and eliminates the need to enrich the fuel mixture at all. Direct injection uses much higher fuel pressures, and the heat of the combustion chamber. This improves fuel economy, warmup times (for cabin heat) and engine performance.
    johnlgalt likes this.
  17. dibblebill

    dibblebill Well-Known Member

    My TDI seems to get significantly lower MPG in the winter than summer... But I stil average 35-37mpg with city driving. I will say this- its MPG counter does seem to factor in idle and stopped time.
  18. nickdalzell

    nickdalzell Well-Known Member Contributor

    i never saw any gas guzzler heat up any faster than a modern EFI-equipped car or truck in my experience. in fact, it took 10 or more minutes before the heater would work in my Fifth Avenue, Dodge Ram, AMX, or any other carburetor-equipped vehicle i owned. i think that is due to older cast iron engine blocks in older vehicles, and more newer vehicles have aluminum blocks/heads and warm up a lot faster than their cast iron predecessors. my Bonneville is warmed up pretty fast, and the electronic climate control won't even turn the fan on until it is warmed up properly.
  19. Speed Daemon

    Speed Daemon Disabled

    Same thing with mine. Those things aren't precision instruments by a long shot. OK for highway MPG on long trips, but that's about it.
  20. Speed Daemon

    Speed Daemon Disabled

    Just the sheer mass of the motors tells the story. The old big block V-8 motors weighed in at well over 600 lbs., which is 2-3 times as much mass as the average 4-banger in most new cars. The catalytic converters make a lot of heat too.
  21. Gmash

    Gmash Well-Known Member

    Diesel engines take longer to warm up than gas engines, and all motors are most efficient when they reach normal operating temperature.
  22. dibblebill

    dibblebill Well-Known Member

    I know they take longer. My car's mileage counter has proven accurate when compared to the old gas tank fill-up calculation.
  23. big_z

    big_z Well-Known Member

    Another thing is the front window defroster. It runs your air conditioner to dehumidify the air, then heats it and directs it to your windshield and side windows. If you run this as your standard heat (in some conditions, I have to on my Civic, as I can't keep the windows clear otherwise) it's basically like driving with the A/C on on a summer day.
    Speed Daemon likes this.
  24. nickdalzell

    nickdalzell Well-Known Member Contributor

    since the A/C compressor is locked up tighter than a drum on my Bonneville, i do not think that problem concerns me :)
  25. SiempreTuna

    SiempreTuna Well-Known Member

    I think the affect of temperature on the volume of the fuel could have something to do with it .. though I don't know what!

    The amount of gas you get in a 'gallon' changes depending on the season / temperature and your region. This is to compensate the gas company for the fact that, in the cold of winter, gas contracts so you would get more in a gallon and in the summer it expands, so you would get less.

    This is entirely legal, however I suspect the calculations somehow end up favouring the gas companies - call me a cynic ..

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