[laperacfo]

Lately I have been getting terrible gas mileage. It's around 17-18mpg. Last night it even dropped to 8mpg. I was idling for a while. Most of my driving now are city. I swear that I saw my gas needle gauge go towards the E side slowly.

[maswastage]

If you're driving mostly city, 17-18mpg is actually quite good.

[rault18]

I get about 18 20 in the city...remember there are TONS of factors that determine ur mpg

[chumley]

Originally Posted by rault18

I get about 18 20 in the city...remember there are TONS of factors that determine ur mpg

That's about what I get around town.

[vasillalov]

Folks,

Thats normal. It happens every winter. Here are the reasons:

1. The air is colder. Colder air is thicker. Thicker air creates more drag which means you need more fuel to keep the same speed. Air at 0 degrees Celsius (32F) is about 30% denser than air at 30 degrees Celsius (90F).

2. The air is colder. Colder air is thicker. Thicker air contains more oxygen molecules per volume of air. Your car computer has to maintain proper Air-To-Fuel ratio which is more technically true to say proper Oxygen-To-Fuel ratio. The more oxygen goes into the intake per unit volume of air, the more fuel the engine will need to maintain proper AFR ratio. The way the ECU knows what's going on is via the oxygen sensors. That's one reason why you should not be using an O2-simulator.

3. Winterized fuel is now sold everywhere. Winterized fuel has less energy (BTUs) per unit volume compared to non-winterized fuel even though the octane level is the same. Even if you live in Florida, you still might see winterized fuel there because of people traveling north.

4. Check your tire pressure. In the cold, air shrinks. You can loose good 2-8 psi in the tires on a very cold morning. This will affect your fuel efficiency.

To help "restore" some of the lost fuel efficiency:

* Check your tire pressure.
* Slow down a bit while @ cruising speed in cold weather.
* Empty the trunk. Carrying 2 dead bodies in your trunk will definitely eat your fuel quickly.
* Check your air filter and replace it if needed.

[laperacfo]

Thanks. Just the answers I was looking for.

[tamim]

city 22/23 mpg ... highway 30-33mpg ... Just came back from Ithaca to mississauga .... 33.1 mpg

[jopa489]

Originally Posted by vasillalov

Folks,

Thats normal. It happens every winter. Here are the reasons:

1. The air is colder. Colder air is thicker. Thicker air creates more drag which means you need more fuel to keep the same speed. Air at 0 degrees Celsius (32F) is about 30% denser than air at 30 degrees Celsius (90F).

2. The air is colder. Colder air is thicker. Thicker air contains more oxygen molecules per volume of air. Your car computer has to maintain proper Air-To-Fuel ratio which is more technically true to say proper Oxygen-To-Fuel ratio. The more oxygen goes into the intake per unit volume of air, the more fuel the engine will need to maintain proper AFR ratio. The way the ECU knows what's going on is via the oxygen sensors. That's one reason why you should not be using an O2-simulator.

3. Winterized fuel is now sold everywhere. Winterized fuel has less energy (BTUs) per unit volume compared to non-winterized fuel even though the octane level is the same. Even if you live in Florida, you still might see winterized fuel there because of people traveling north.

4. Check your tire pressure. In the cold, air shrinks. You can loose good 2-8 psi in the tires on a very cold morning. This will affect your fuel efficiency.

To help "restore" some of the lost fuel efficiency:

* Check your tire pressure.
* Slow down a bit while @ cruising speed in cold weather.
* Empty the trunk. Carrying 2 dead bodies in your trunk will definitely eat your fuel quickly.
* Check your air filter and replace it if needed.

Great post. Should be required reading for anyone complaining about MPG.

[JmanND]

Be happy, I get like ~16mpg mostly city driving here in LA. Always getting stuck in traffic kills my mpg. I got great gas mileage bringing my car out here though (~31)

[Gart]

Originally Posted by vasillalov

2. The air is colder. Colder air is thicker. Thicker air contains more oxygen molecules per volume of air. Your car computer has to maintain proper Air-To-Fuel ratio which is more technically true to say proper Oxygen-To-Fuel ratio. The more oxygen goes into the intake per unit volume of air, the more fuel the engine will need to maintain proper AFR ratio. The way the ECU knows what's going on is via the oxygen sensors. That's one reason why you should not be using an O2-simulator.

Sorry, but have to call foul on this idea. If this were true, aircraft would fly as close to the ground as possible to avoid flying in the colder, thicker air at altitude to avoid drag.

If its colder in Denver than it is in San Diego, it doesn't mean that there is more oxygen in Denver at the time compared to San Diego.

I will agree to a point that colder air is more dense, but not as extreme as you think. Atmospheric pressure has more to do with density than temperature.

Personally, I think it is more likely to be a winter blend of gasoline.

Gart

[Kilgore Trout]

Originally Posted by jopa489

Great post. Should be required reading for anyone complaining about MPG.

I agree. In fact, I am on my way out to the garage to remove the dead bodies from my trunk right now.

[John 070]

I was concerned about what mine would be after a software upgrade, but sorry, hit 31 mpg on the highway today. I'm really not sure but I'm willing to bet my car IS slower after the software upgrade. I'd rather have the same mpgs and stuff not broken and the car not dying on the highway while giving up some performance...

[335iSSA]

Originally Posted by Gart

Sorry, but have to call foul on this idea. If this were true, aircraft would fly as close to the ground as possible to avoid flying in the colder, thicker air at altitude to avoid drag.

If its colder in Denver than it is in San Diego, it doesn't mean that there is more oxygen in Denver at the time compared to San Diego.

I will agree to a point that colder air is more dense, but not as extreme as you think. Atmospheric pressure has more to do with density than temperature.

Personally, I think it is more likely to be a winter blend of gasoline.

Gart

Vasillalov is correct. As you go to a higher altitude the air gets less dense thus airplanes would want to fly as high as possible to fly through a less dense atmosphere with less resistence. This is why when folks who live at sea level go to Denver they get out of breath climbing a few flights of stairs. Also why Lance Armstrong trains at altitude to obtain maximum efficiency in his cardio system.

[AWD Addict]

Originally Posted by Gart

Sorry, but have to call foul on this idea. If this were true, aircraft would fly as close to the ground as possible to avoid flying in the colder, thicker air at altitude to avoid drag.

If its colder in Denver than it is in San Diego, it doesn't mean that there is more oxygen in Denver at the time compared to San Diego.

I will agree to a point that colder air is more dense, but not as extreme as you think. Atmospheric pressure has more to do with density than temperature.

Personally, I think it is more likely to be a winter blend of gasoline.

Gart

Call foul if you like. It won't make your post any more correct. Unless the OP was driving at dramatically varying altitudes, your post has no relevance.


Winter gas has an effect, but not that dramatic. I see roughly 25 miles difference / tank between the oxygenated crap and summer gas.

Great post vasillalav

[JakeN5]

Guys, don't rely on that silly gauge or trip computer for mileage info. Why don't you just divide the total number of miles driven after a near empty tank by the number of gallons it takes to fill up the tank, and you'll know your mileage? Don't forget to reset the odo to 0 after the last full fill-up to get an accurate distance driven.

[tooshay]

Originally Posted by laperacfo

Lately I have been getting terrible gas mileage. It's around 17-18mpg. Last night it even dropped to 8mpg. I was idling for a while. Most of my driving now are city. I swear that I saw my gas needle gauge go towards the E side slowly.

I wish I was getting 17 mpg in traffic/city driving. I only get 13 mpg and have to fill up every few days. I guess you have to pay to play.

[detrick]

I get 21 city and 31 highway in my 335i. If I set the cruise control to 75 mph, I get 32 mpg. We are 600 feet above sea level.

[Zero.7]

I'm actually ecstatic that I'm getting b/t 18-21 mpg with my 335i. Coming from a B7 RS4, I was averaging 13-15 mpg (and that's when I'm not driving too spirited!) I realize that's not comparing apples to apples, but you get why I'm happy to average 18.5 mpg now.

[vasillalov]

Folks,

I admit. I made a mistake on the numbers of the density of the air. It is not 30%. It is 10%.
Source: http://en.wikipedia.org/wiki/Density_of_air

It shows that air at 0 degrees Celsius is 10% thicker than air at 30 degrees Celsius.
It also shows that air at -20 degrees Celsius is whole 17% thicker than air at 30 degrees Celsius.

Basically, the air density between a very cold winter morning and a hot summer day is 17%. For me, that's a significant difference if you are cruising at 80 mph.

[bsamoul]

just curious, do you think having the sunroof vented vs closed makes any difference?

[John 070]

Originally Posted by JakeN5

Guys, don't rely on that silly gauge or trip computer for mileage info. Why don't you just divide the total number of miles driven after a near empty tank by the number of gallons it takes to fill up the tank, and you'll know your mileage? Don't forget to reset the odo to 0 after the last full fill-up to get an accurate distance driven.

That "silly gauge" is accurate. If I use the same pump and do not attempt to overfill, it is exact to 0.1 mpg when compared to doing the division myself.

I think what people often miss is that use a statistic it should be reliable and valid. Without driving 100+ miles, most numbers are not repeatable.

[John 070]

Originally Posted by detrick

I get 21 city and 31 highway in my 335i. If I set the cruise control to 75 mph, I get 32 mpg. We are 600 feet above sea level.

This is very typical as far as mpg's with a 335....