The Oil Drum | Recent VMT and Fuel Economy Trends

48 comments on Recent VMT and Fuel Economy Trends

Comments can no longer be added to this story.

| Show without comments | PDF version

Luis de Sousa on June 12, 2006 - 4:57am

Interesting post, thanks again Stuart.

The seasonal signal we discussed at length last time, and the general feeling is that fuel economy is poorer in the winter because cars are less efficient in the cold.

My common sense leads me to think that this is wrong. Once warmed-up an internal combustion engine should provide more horse-power in cold weather. A more compressed mix that goes into the combustion chamber and the wider temperature difference between the engine and the environment should allow it to produce more work.

Those seasonal fluctuations on fuel efficiency are a bit strange, but can be explained by the different driving patterns during the summer. Although a car has a better fuel efficiency at `not-so-fast' speeds (80 - 90 Km/h), it will be far more efficient on a Highway at 120-140 Km/h, than on traffic at 60-70 Km/h.

Beyond looking at efficiency we must also look into inefficiency: it is during the stop-and-go that a car has the worst MPG or L/100Km figures. Since during the Summer we are mostly on highways and freeways we are avoiding the usual traffic jams and traffic lights, that bring the car's efficiency down.

Comments can no longer be added to this story.

odograph on June 12, 2006 - 8:14am

In the Prius forums I hear a number of people saying that "winter gaslone blends" keep their mileage down during that season. I wonder how oil industry types respond to that ...

(In California I haven't noticed a lot of seasonal variation.)

Comments can no longer be added to this story.

Bryant on June 12, 2006 - 2:30pm

Indeed. My diesel also gets about 10% fewer miles to the gallon during the winter; from 24-25 mpg during the summer to 22-23mpg during the winter.

Comments can no longer be added to this story.

Robert Rapier on June 12, 2006 - 8:18pm

In the Prius forums I hear a number of people saying that "winter gaslone blends" keep their mileage down during that season. I wonder how oil industry types respond to that ...

Formulations do change in the winter. The most significant change is that the butane in the gasoline pool increases from about 2% to about 10%. I suppose this could affect fuel efficiency.

Comments can no longer be added to this story.

RJB on June 12, 2006 - 10:34am

My Civic hydbrid consistently (over three years of driving) gets about 10% worse mileage in the winter than in the summer, with driving habits essentially the same, 80-90% highway, keeping the speed to 65-70 mph.

Comments can no longer be added to this story.

mcain6925 on June 12, 2006 - 10:52am

My non-hybrid Civics have always displayed much the same behavior, although the drop-off is more like 5% than 10%. Part of it here in the Denver metro area is that oxygenated fuel (all ethanol additives now that MTBE is gone) is required during the winter months. At least some of the gas stations put up signs in the spring when they receive their first load of ethanol-free gasoline. What effect does cold weather have on the electric side of the hybrid power train?

Comments can no longer be added to this story.

ET on June 12, 2006 - 12:41pm

Ditto the Honda Insight. Honda dealer 'mileage specialist' says it is because of the winter fuel formulation.

Comments can no longer be added to this story.

LJR on June 12, 2006 - 10:57am

Common sense doesn't work here. The temperature that's important is at the cylinder walls. That temperature is governed by the thermostat. Doesn't matter what the "outside" temperature is. Thermodynamically the outside temperature is the cylinder wall temp.

Don't mistake performance for efficiency. Cold air is more dense so more air can pack into the cylinder chamber. Fine. It's still colder than summer air and the energy to warm it has to come from the compression stroke. That means the temperature at ignition is lower. Since it is denser there can be more fuel injected and an overall higher power output. That does not imply the efficiency is greater because it takes more fuel to get that additional power. It's more complicated than it seems.

Comments can no longer be added to this story.

Luis de Sousa on June 13, 2006 - 3:27am

It makes sense.

Comments can no longer be added to this story.

kjmclark on June 12, 2006 - 12:19pm

I suspect that if you stuck your head out of the window at 55mph on a winter's day and on a summer's day, your common sense would be quickly corrected! To save your neck, you might try sticking your hand out of the window instead of your head :-)

Wind drag is the biggest factor slowing wheeled motor vehicles above 30mph or so. Wind drag increases with the square of the speed, while tire drag is pretty much linear with speed, and the various forms of engine drag vary with engine speed. Air in winter is significantly denser than air in summer, so drag increases in winter.

On someone else's point, people may drive more on highways in the summer, but there are far fewer orange-barrel obstacle courses in the winter. I suspect that road construction has a mixed effect on summer motoring efficiency. On the one hand, there are more traffic backups due to construction. On the other hand, motorists are forced to travel at lower speeds in construction zones, which in some cases will mean they are getting better mileage!

Comments can no longer be added to this story.

48 comments on Recent VMT and Fuel Economy Trends

Comments can no longer be added to this story.

| Show without comments | PDF version

“The aim of every political constitution is, or ought to be, first to obtain for rulers men who possess most wisdom to discern, and most virtue to pursue, the common good of the society; and in the next place, to take the most effectual precautions for keeping them virtuous whilst they continue to hold their public trust.”

—James Madison, FEDERALIST #57 (1787)