Mileage Thread #4000001 - Winter Gas Mileage Myth

In every mileage thread, I see people blaming "Winter Gas" for part of the mileage drop during the season. Technically speaking, winter gas has almost the same energy content, it is roughly 2.5% less energy, (we'll leave the math out, but that's pretty close) at most, if more Ethanol is added. Fuels where I live have ethanol all year,and the proportions do not change much at all seasonally, it's more of an availability issue.. Winter fuel is generally formulated to vaporize easier, allowing easier starts, and combustion. That is why summer fuel is different, to prevent vapor lock. A 2.5% loss in energy, may affect fuel mileage at most 2%, which is generally not measurable.
However, as read on a ScangaugeII, a warm start idle will burn around 1.25 LPH (liters per hour). A cold start at 5C will start off around 5 LPH, and for the first 10 miles or so, mileage is poor, before the mixture leans back out, fluids heat up, and my mileage goes to it's normal highway mileage.
So if starting cold at 5C uses 4 times the fuel, -20 or colder start must use closer to 10+ LPH at idle, and takes around 20 miles to go to it's winter normal mpg number. So just the act of starting it, never mind idling, uses lots of extra fuel. Denser air, emissions considerations, you name it, it's working against fuel economy. Shorter days using lights more, blower fan cranked, and on and on//
It's not the gasolines fault.

 

Yep... I track mileage daily on trips back and forth to work. My mileage drops steadily as the ambient temp goes below 45ºF, even with the same gas in the tank. February and March were interesting because we'd have relatively cold stretches (temps in the 15-20ºF range) followed by days in the 50-60ºF range. And now that we've been up in the 80s-90s lately the change has been dramatic. Maybe there's different blends elsewhere but where I live the variations are all about how much longer it takes my truck to warm up.

 

You are on the right track on cold start fuel consumption. Starting your vehicle at -20c will not make you use double the fuel at startup. It simply takes longer to get to oprating temperature, so you'll be in open loop mode longer. Also, many of the same people that complain about the drop in fuel economy happily let the vehicle idle in the driveway in the morning to warm up the interior. Due to this, the car sits idling away very large amounts of fuel going nowhere. This is where the largest portion of your losses add up.

 

I noticed that if it is above 50F when I walk out the door my mpg jump a couple points over days when it is in the mid 30's

I believe it all has a lot to do with how long it takes the engine to reach normal operating temp

 

its the 4.0 gas gusler thats the problem

 

So would it be smarter to have the truck warm up before driving it for like 10 minutes if i'm just going on a 15 minute drive to school? Is it that much of a difference in gas mileage?


(I don't give a shit about gas mileage, I just want to see)

 

Nope. Just start it up, put on your seat belt, and get going. By the time you have your seat belt on, oil will be flowing through the engine.

 

Ok so by the time you actually get going the engine will be warm enough that it wont actually matter. Cool

 

Your engine will still be cold. You just won't be wasting fuel letting it warm up in the driveway.

 

This.

Even with the engine not up to operating temperature you'll be better off mileage-wise driving it. Regardless of engine temperature, sitting still nets you 0 mpg

 

In 10 minutes of idling, you would use as much as 1 liter of fuel, where I live thats 1$. Not to mention, that's only warming the engine, not the T-Case, or axles, or any of the many parts that have lube in them.

 

I have a question. The manuel says to allow 10 seconds of warm up. What about the trans? That damn thing lags until warm, that eats up my fuel. I understand the idleling issue staying parked nets you 0mpg. But for those of you in the extreme cold would it save you some fuel to warm the trans some?

 

The 10 seconds of warmup is just to get oil pressure up and oil flowing throughout the system. The rest of the drivetrain will not warm up at all until you are moving, so no savings will be realized letting the truck warm up in the driveway.

 

I think you misunderstand how open loop fueling takes place. It provides a fixed ratio for enrichment, (not a fixed amount of fuel) this ratio goes beyond calculating the amount of air , it factors in density of the air, and the temperature. But that is only half the reason for the extra consumption; you are also not factoring the higher idle speed, which is elevated temporarily. You are correct in stating the temperature outside is a large factor in the time this occurs. But there is still high idle, fuel enrichment, fuel cutoff delay, and torque converter lockup delay, and other factors that are affected by a cold catalytic converter, and not just temperature.
But your missing the point, the LPH of fuel used is incrementally higher for every degree the intake air is cooler, idling or driving. And at different temperatures, it is noticably different, with the ScangaugeII, which was the whole point. The thickness of the oil is another reason why, I could go on and on.
In theory there is no difference between theory and reality, right.

 

Within 1/4 tank of filling up with winter/summer gas - I can tell a very noticable difference (3 days worth = 1/4 tank) that I have a different gas blend. In using my scangauge....I get approx 16-17mpg in winter and 18-19mpg in the summer - with similar weather and similar driving styles.

My husbands Acura MDX gets the same gas mileage all year long regardless.

So, something about these trucks just don't work the same in summer/winter gas mixtures for whatever reason.

 

WRONG

At startup in the cold weather your air/fuel ratio is in the 2:1 neighboorhood. You cannot argue this point.
When the vehicle is warm, the fuel ratio is 14.7to1 regardless of temperature. You can talk all you want about the differences in fuel, and yes there are differences, but the bottom line is the fuel ratio required to reach operating temperature is much different.

Most cars enter closed loop in 20 seconds or less on newer vehicles, regardless of outside temp. Closed loop is determined by O2 sensor temperature, NOT ambient temp. Most lambda style air/ratio sensors (not zirconium dioxide type) can warm up in 20 seconds or less.

Now, if you want to factor things like the volatility of the gas, it is definatly easier to burn fuel in colder climates because the RVP is higher (Reid vapor pressure). BUT - The air is colder, and therefore thicker, in a cold climate, which can effect aerodynamics. Also, colder air contains more O2 by volume, and the computer will counter this by increasing injector pulse width, therefore consuming more fuel.

In short, your vehicle will use more gas in the winter.
Paul

 

I won't argue. I'll just tell you flat out that it is physically impossible to get an air to fuel mixture of 2 to 1 to burn.

Most engines like a ratio of 16:1. 14.7:1 would be the rich mixture, not the warm.
YMMV

 

No veggies in our gas up here north of the boarder. My mpg drops in the winter because I have to let my truck run for a while while I scrape the ice off the windows so I can see where I'm going