As the weather gets cooler, I have found my mileage has been getting worse. I can think of 2 reasons,

1/ The cooler more dense air is burning hotter and the ECU is compensating with more fuel. I am certainly getting more power so it would make sense that I would need to burn more fuel.

2/ The cooler air, and more power is causing me to drive harder and push the car harder than I would normally. I don't think I have been doing this but its certainly possible.

I know there was a thread on CAI and fuel economy and it didnt look like there was much change, however if my #1 is true then there should be change. In theory #1 would seem to make sense to me. All I know is I am doing slightly worse per tank than I was 1 or 2 months ago.

Anyone else notice a change in fuel economy as the temperature dips?

 

It is also caused by the extended time to get to operating temperature where the fuel economy is best.

 

I havn't noticed a difference yet but wosre MPG is definently around the corner. I'd guess #1.

 

Interesting. It only takes a couple minutes at most. Do you think this would have a noticable impact?

 

Don't the refineries reformulate the gas for winter and summer? Would that cause a difference in mileage?

 

Yes they do reformulate the gas. Actually in the winter they oxygenate the gas which make th gas ingnite quickly and burn clean and faster. Here read this. Just my

http://www.energy.ca.gov/gasoline/ty..._gasoline.html

 

I have always noticed a change in gas milage between summer and winter. With my old Integra I would get 220 in the winter and 280 in the summer. I was told that it has to do with the anti-freeze mixture in the fuel that prevents the fuel from freezing. I don't think it's the cold air.

 

FDL - Good thread. I'd agree with both of your reasons.

I have always believed that cars run better in the cooler weather. I know people take their cars to the drag strip when it cools down for optimal performance. (it would be interesting to know what is truly the best temperature, because I am sure there is a point where the colder it gets doesnt help). Colder air is more dense so requires a richer mixture - leading to better performance.

I also push my car a little harder when it has that new pep in it, so that's probably part of the reason as well.

My

 

I just did the same thing, so I'm going to look for some info to back my post, but we need to make sure we're not just spreading rumors. I don't know if they add an anti-freeze mixture to the fuel in the winter. I may be mistaken though.

 

Possibly, but I doubt that the gas right now has any anti-freeze mixture in it yet, so that wouldnt explain the difference today.

 

Gasoline and Freezing

My other thought is that regardless of the temperature outside, the TSX is going to warm up to the same temperature (which uses a lot of gas). I think once the car has warmed up, the performance should be roughly the same as other temperatures. This is why people used to put colder thermostats in their cars - to allow their engine to run cooler and thus run a little richer.

 

I am not 100% sure. I would check with Mobil or Exxon. My friend told me it had to do with the additives. You are not burning 100% gasoline fuel in the winter, but a mixture. The mixture (additive) is what burns faster. We are using Winter fuel.

 

Have you checked your tire pressure recently? I just noticed that mine is 4lbs lower than normal, because I haven't checked since temps started dropping.

I've always gotten lower gas mileage during the winter. Usually starting October and going thru April. My fiance usually drives the TSX and her driving habits never change, but the mileage has definitely dropped, but that may be because of the tire pressure. . .

 

The more I think about this the more I think it's the fuel.

My reason is the following:

In the summer there is a greater demand for fuel, because more people drive. So gasoline companies make good money. In the winter there is less driving (trips etc). So how does a gasoline company make up the difference? They can't just jack up the price on gasoline with less damand. They need to find a clever way to sell more gasoline or lower the cost of gasoline. You can do both by mixing in an additive. One, you are burning fuel faster with the additive, so you are fueling more often. Two, you are lowering your cost to produce the gasoline, because you are selling less gasoline.

I think this fits nicely on a supply and demand curve, but then again I didn't get a good grade in econ 101 and 102.

 

You get an "A" in Conspiracy Theory 101. .....BTW, I think you might be right.

 

For a relatively short commute, definitely. The engine runs much richer during the warm up stage so the longer that lasts the worse the milage. It's real obvious on my bike that takes at least twice as long to fully warm up in the winter as summer - even with it in the garage to start.

 

Like others have pointed out, it takes longer for engine to reach the operating temp when cold. The dash gauge is merely an indicator for the driver to reference, but not where the ECU is pulling the actual coolant temp info from. The ECU will continue to richen the a/f mixture in open-loop mode until the coolant temp has reached to operating temp, then it switches to closed-loop mode. If you have a scantool and a a/f guage connected, you can easily tell when the ECU exits out of open-loop and at what coolant temp.

 

Good thread fdl. I think for the most part, you're right on with the reasonning. And JTso is right as well, but that would account for only a minor part of the problem, especially if your drive a lot of highway like I do.

The weather now is more between 0 and 10 and I have already started experiencing a good 8-10% drop in mileage, and I do long road trips.

 

Yeah same here.

My second fill up after the temperature drop however has been normal.

 

The fuel addative theory is sound, although you are never buying 100% gasoline, there are always addatives and in the winter time their proportions shift. The prime culprit is Ethanol/Alcohol which is added to prevent fuel line freezing, improve the vapourization characteristics at low temperatures, as well as a whole other reasons which in the end help them justify watering down the fuel we already pay too much for.

I currently take my WRX to work each day, and it requires no less than 94 octane (a retune which is costing me $$$ in fuel! ) which has already 10% ethanol. On average during the summer I get roughly 530km to a 50L tank, in the winter I lose more than 10% of my mileage and my exhaust gas temperatures jump 50°C.

Power wise things balance out, the watered down fuel doesn't burn as well, but the low temps and virtually 0% humidity work well with a turbo and intercooler. I definitely notice the difference in power between summer gas at low temps and winter gas at low temps.

As has been mentioned, engine temperature is a huge player in fuel economy. Cold oil is much thicker than hot oil, and therefore robs more power. Warm up fuel mixtures for modern cars are actually quite lean compared to cars 5 years ago, this is because emissions are critically controlled while your catalytic converters come up to temperature. That said it takes more energy to heat something from -20°C to +100°C than it does to heat something from +20°C to +100°C, and since your engine's only source of energy is the fuel you're feeding it, logically it would take more fuel... alot more fuel.

You engine also knows the incoming air density and adjusts the mixture accordingly. Denser air requires more fuel, which intern will give you more power but more power doesn't necessaryly translate to the same amount of fuel for the same amount of work.

Adjusting your tire pressure and making sure you are running the proper weight of oil are about the only things you can do to combat the mileage losses.

-SWRT

 

C&D had an interesting write up about this in a recent issue. It was one of the early in the magazine one pagers. Can't find it on the site but I read it and it was pretty good. One thing it mentioned, which I don't think has been mentioned here, is that colder, dense air, which contains more oxygen and therefore helps the engine performance, also creates more resistance through which the car must travel, which then hurts the fuel economy. It said the best outside temps for fuel efficiency are between 50-70 degrees I believe. Wish I could find the article but that is one point I remember.

 

In my case (Colorado), the MTBE that gets put in the gasoline to "improve" air quality causes lower gas mileage, around 1-3 mpg. There's been several writeups/investigations, on the MTBE additive, and the conclusions have pretty much all been the same, the MTBE added does't improve air quality enough to offset the additional fuel burned. Couple of the investigations dug a bit deeper, and started finding some very dirty politics involved..... So, for me at least, it isn't the cold air, but the MTBE that robs my TSX of it's mileage.