Warmer Temps Give Better Mileage?

[Vimy101]

Something you're all overlooking.....

 

Colder air IS denser and DOES contain more O2 per unit measure. Since you're putting more air into the same space, you have to add MORE fuel to accompany the larger O2 volume.

 

Then ECU should be compensating by providing a leaner mixture for a more complete burn and colder temps should allow for timing advance.

[Warnz]

I'm in Chicagoland too. Just bought my LGT Easter weekend. When I did a test drive, I noticed the avg MPG was 13. something. I was concerned, but undeterred. Somebody must have been jumpin on it.

 

I was averaging about 19.5 until I had 30K service done. Now I'm getting about 22 MPG. Must be the summer gas all right.

 

My other car is a '99 M3 with a CAI. I've noticed at about 55 degrees it runs the best. Just breaths very well at that temp.

[ZinFreak]

Then ECU should be compensating by providing a leaner mixture for a more complete burn and colder temps should allow for timing advance.

 

If you didn't care about the catalytic converter that would be reasonable to achieve better mileage. Running slightly lean of peak will yield better fuel efficiency.

 

However, my understanding is that automotive manufacturers bias the AFR (air-fuel mixture ratio) towards stoichiometry in order to extend the life of the catalytic converter, which they have to warranty to 8 years or 80,000 miles BTW.

 

See http://www.epa.gov/OMS/consumer/warr95fs.txt

[ncretter]

"towards stoichiometry"? Isn't that like saying, "towards math"? (I don't remember my chem very well, always prefered physics)

[mwiener2]

stoichiometry is an ideal air fuel mixture.... in cars.. 14:1

[just_phil]

I too noticed seasonal decrease/increase in gas mileage, but I don't blame it entirely on winter gas or cold air. I think in my case it's more related to which tires I run. When I switch to my LM-25s, I immediately notice a drop, and when I switch back to RE-92, I notice an increase. RE-92s FTW

[hyteck9]

I think everyone is a little bit right...

 

Increased drag from cold weather, increased rolling resistance from under-inflated tires, extra fuel to match dense air, engine warm up, etc..

 

They all play a part. Another thought...

 

I think the engine warm up cycle is particularly brutal though. Not only is the ECU trying to do its part to warm things up, but the close tollerances of today's engines and drivetrains are made even closer by the -15'F temps... not to mention the accessory belts that are now tight as daisy dukes on Rosanne Barr from the cold. Tons of friction and resistance that is eating up your torque until things warm up and loosen up.

 

Just_phil, I agree with you too. winter snow tires with large sipes and lugs that have low rolling resistance will show noticable milage increases over sticky performance summer tires... usually 5 - 6 MPG.

 

I usually do about 27 mpg in the winter and 32.5 in the summer.

[Vimy101]

^ Larger lugs have increased tread squirm resulting in increased rolling resistance which reduces fuel efficiency.

 

When I started this thread, I was refering to a thoroughly warmed engine and drive train still getting worse mileage on a given gas formulation when the weather gets colder. The temps have dropped into the 50s in these parts for the last few days and my mpg has dropped off once again.

[just_phil]

Just_phil, I agree with you too. winter snow tires with large sipes and lugs that have low rolling resistance will show noticable milage increases over sticky performance summer tires... usually 5 - 6 MPG.

 

No, I meant it the other way around. Winter tires give me worse mileage, around 22.5 mpg, while the all-seasons 24+. I was implying that these LM-25s have higher rolling resistance, not lower, and that the OEM RE-92s were picked by Subaru (likely) because of their mpg 'performance'.

[Vimy101]

^ +1