School buses + Colder Temps = MPG Nosedive

[RayCJ]

A few weeks ago, the school buses made their presence known as kids went back to school. At the snap of a finger, morning commute times increase by 20%. A couple weeks after that, temperatures are down about 30 degrees (F) as we head into winter. Net result: Per-tank fuel mileage goes from 36 MPG down to 31 MPG. -Can't wait for the next big drop in temperature after Thanksgiving.

FYI: For every 10 degrees (Fahrenheit) temperature drop, air density increases by about 1%. (please don't nit-pic, it's an approximate rule of thumb with plenty of caveats).

Yeah baby... really looking forward to spring already!

Ray

 

[14Accent]

Well, my Cadillac is only averaging 15.6 right now and that's with nice weather! Can't wait to see what snow tires and long cold city commutes do for it.

The Volt, on the other hand, is pretty much unaffected. Yes the battery range blows in the winter but it's plenty to get to work and back.

 

[Danno]

Winter gas with less BTUs per gallon is in the system as well.

 

[RayCJ]

Originally Posted by Danno
Winter gas with less BTUs per gallon is in the system as well.


I always get that confused and can't remember if winter of summer gas has less BTUs... If it's winter; well, there's another nail in the coffin.

 

[Kruse]

I'm pretty sure that the winter blends of gasoline make it less expensive per gallon to manufacture, although that hasn't been the case this year.

 

[Quest]

Check your thermostat and replace if necessary.

Q.

 

[Audios]

I thought the drop in winter performance was only from the fuel, has less octane molecules from the additives per ounce so is technically less powerful. Cooler air is more dense with oxygen, so better performance.

 

[RayCJ]

Originally Posted by Quest
Check your thermostat and replace if necessary.

Q.



Thermostat is fine (brand new car) and also, I've observed this same dip in fuel economy for decades with many different vehicles. I would estimate an 20% increase in stop/go traffic for first 5 miles and last 3 miles of my commute. I suspect that is the main culprit. This is due to all the local colleges back in session (kids driving to school) and school-buses causing havoc on local traffic.

Originally Posted by Audios
I thought the drop in winter performance was only from the fuel, has less octane molecules from the additives per ounce so is technically less powerful. Cooler air is more dense with oxygen, so better performance.


The winter/summer blends do indeed impact BTU content of the fuel. Air density and oxygen content are 2 different things. The percentage of oxygen remains basically the same and the MAF sensor compensates the fuel mixture for optimal and constant A/F ratio. Efficiency in that regard remains the same. When between 40 MPH up to about 60 MPH resistance is 1/2 x (Air Density) x (Surface Area) x (Drag Coefficient) x [Relative Velocity/3.6]Squared. Therefore, every 1% increase in air density is an immediate 1% decrease in fuel economy. Over 60 MPH this formula is not really accurate and resistance shoots-up considerably more.

This same problem is seen with long distance rifle shooting all the time. All serious competitors will notice a vast drop in point of impact (due to lower terminal velocity) using the same ammo in winter vs summer. It's due to the increase in air density but since the speeds are so much higher, the effect is much more noticeable. On a car, the air density difference between winter and summer is probably 5 to 10% depending on region. That's a considerable difference in fuel economy.

Don't forget tire inflation. I keep a close eye during these transitional times but many do not. If memory serves, for every 10 degrees (F) there is a 1.2 psi change in tire pressure.


Anyhow, I guess the point is that I'm not crazy about winter and less happy about all the additional school commuters.

 

[jongies3]

Yep I'm already ready for spring. I hate winter, snow and ice so much! I've never understood why people are so in love with it, there's nothing fun about shoveling snow, layering up, being cold, slipping and falling on the ice, corrosive chemicals used on the roads, higher utility bills, etc.

 

[maxdustington]

Originally Posted by jongies3
Yep I'm already ready for spring. I hate winter, snow and ice so much! I've never understood why people are so in love with it, there's nothing fun about shoveling snow, layering up, being cold, slipping and falling on the ice, corrosive chemicals used on the roads, higher utility bills, etc.
After a hot summer, you cannot wait for it to cool down come autumn. After a cold winter the first signs of spring brings such happiness and hope for the future.

 

[Nick1994]

Can't wait for winter here, my mpg also goes up 10-20%.

 

[Colt45ws]

Originally Posted by Nick1994
Can't wait for winter here, my mpg also goes up 10-20%.

From finally leaving the A/C off?

I stopped tracking MPG a long time ago. I pretty much get 15MPG all the time, or 20MPG on the highway. I dont commute far enough for the car to fully warm up. Gonna see about getting the car plugged in to the block heater again. That will help.

 

[Nick1994]

Originally Posted by Colt45ws
Originally Posted by Nick1994
Can't wait for winter here, my mpg also goes up 10-20%.

From finally leaving the A/C off?

I stopped tracking MPG a long time ago. I pretty much get 15MPG all the time, or 20MPG on the highway. I dont commute far enough for the car to fully warm up. Gonna see about getting the car plugged in to the block heater again. That will help.

Well, not leaving it off. Just off for part of the time

No idling trying to cool the car down, and probably something about reasonable temps.

I keep track of every fuelup in Fuelly, also other vehicles in the family for maintenance records:

http://www.fuelly.com/driver/nickfancher1

 

[RayCJ]

After searching some recent articles, the answer to my quandary about winter/summer fuel, is that winter blend fuel contains less energy than summer blend. The purpose of the modification is to reduce the volatility (i.e. ease of vaporization) in the summer and slightly increase volatility in the winter. Each State sets it's own rules so the percentage is not set in stone. In two cited examples the percentages were 1.7 and 2.0%. Fuel destined to highly populated areas is doctored-up more than rural areas.

References:

https://newsroom.aaa.com/2013/06/what-is-the-difference-between-summer-and-winter-blend-gasoline/

https://www.bellperformance.com/blog/bid/49483/Fuel-Mileage-Dropping-Blame-The-Winter-Blend-Gasoline


Ray

 

[Vikas]

I have four different vehicles. They *all* routinely drop the mileage between 15-30% during the winter time frame. It is combination of few factors such as different gas formulation, more warm up time, more engine idle time etc.

 

[SirTanon]

Originally Posted by Nick1994
Can't wait for winter here, my mpg also goes up 10-20%.


My MPG is already going up by a rather noticeable amount since the temps dropped back down into the 70's - 80's. With the amount I drive, not having to run the A/C is a big boost.

 

[Vikas]

At first read I was shocked to read school bus fuel efficiency dropping from 36 to 31 mpg