RD400GUY:  In general colder weather means less mileage, but more power.  Anything such as tuneups/filter replacement or cleaning), lighter weight oil, etc. can affect mmileage.  also fuel suppliers change the blend of fuel to the season.  All of these and more, including headwinds, and even slight, barely noticeable changes in road/traffic/driving habits/ and not filling the tank to the same point exactly, can affect the mileage.  

You're undoubtedly spending more time in the warm-up cycle, during which time some warm-up enrichment is taking place (and at a higher rate at the lower temperatures now involved).  40*F is a significant temperature change, and it's probably all the more noticeable if it happened suddenly rather than gradually over a few weeks time.

It also takes energy to splash through standing water and force it out from the contact patches.

Norm

A couple of things come to mind regarding this situation.

Not knowing what your driving conditions are, I wonder if the slip of the wet roads is having a detrimental effect, and, I wonder if your actual driving techniques minute to minute are the same.  In other words, is everyone breezing along comfortably during the good weather, while alternately braking/accelerating while driving cautiously during tenuous wet road conditions.

Remember too that all your oils and lubes, engine, rear end, trans, drivelines, wheel bearings, etc are stiffer in the colder conditions, and that affects gas mileage.

In my part of the country, colder weather, notwithstanding my previous paragraph, is always a gain in gas mileage because the air conditioner is off.  Now there is a horsepower hog.

rmw

Thanks for the replies.  At least I know I'm not nuts.  As for the AC comment, I use the AC even in winter as my car fogs up quite quickly.  To tell the truth, it's very rare that my AC is ever turned off.

I didn't think about the slow down / speed up driving that IS taking place in rainy weather.  The highway is much smoother (in terms of traffic flow) in the summer.

Another interesting factor, I got the oil changed yesterday and the tech mistakenly put conventional oil in instead of synthetic, which I've been running since break-in.  I can't wait to see how that affects things.

rd400guy,

Colder, denser air DEFINITELY affects the drag forces, and hence, fuel usage for automobiles.  There is information available regarding this - just try a detailed search (Google, Scirus) and you should find some information.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Sort of thing that makes the difference for me is having the rear demist and headlights on more.

A.

Even though you have the AC on all winter, (you and my wife) it is drawing much much less work off of the engine in the winter time.  The work comes from the compression of refrigerant, which is not nearly at the same flow rate in the winter running the defrost requirements as it is in the summer trying to cool the universe.

If you have the type of system that merely cycles the compressor clutch, you can definitely hear it hammering on and off all winter (and needing replacement about as often as my wife's) vs seldom hearing it cycle in the summer during the day time.  If it is throttling valves, then know that much less refrigerant is flowing and being compressed.

Regarding the purist arguments of air temperature et al, you have to balance the engine efficiency improvements due to the colder denser air, (and water vapor) vs the losses due to additional drag.  If you are going to get extremely purist, remember too that you have multiple mini inertial impacts with each and every rain drop that you encounter.  Picture hitting all the rain drops you encounter all in one wall of water.  Quite a blow, huh?

If you have a (or have someone who has a) exhaust pyrometer, watch your exhaust temperatures during acceleration.  They are very revealing as to power required during acceleration (multiple times) during winter type driving conditions vs exhaust temperatures at smooth running.  If you remember your college calculus, then just do the second derivative equations that would apply to work done during acceleration.

But, back to the original comment of this post, even at a reduced work load, the AC compressor still adds to the acceleration work load.  (Unless of course, you mash the gas hard enough to make it cut out all together.)

I think I read your post about the oil change mistake.  I wonder if the performance difference is computable during the time you use the conventional oil before you get back to your synthetic.

And, now an anecdotal story as to the importe of lubricant temperatures.  I once returned to the bus from snow skiing and found my buddy there with a tow truck trying to get the thing jumped off (he wasn't an experienced driver, but it was his father-in-law's bus.) I heard them make a start attempt as I walked up, and observed that when the tow truck driver quit spraying ether, it would quit.

Upon stepping up into the bus, I observed that my buddy was sitting sideways in the drivers seat, feet toward the entrance deck.  About that time the tow truck driver, who has his jumper cables hooked up and who had been letting the batteries build up again (it was about 20°F outside), shouted 'try again'.  As he reached for the key without shifting around in the seat, I asked him what he was going to do and when he said 'start it' I said, "not without turning around and dumping that clutch you aren't."

It hacked him off, but he did it, and it started right up and ran just fine.  He very quickly let off of the clutch, and immediately the transmission drag killed the engine again.  Well, the end of the story is that it started right back up with the clutch dumped again, and it took quite a while until the oil in the transmission was heated up enough to even permit the clutch to be let out so that the engine could just sit there and idle.

Moral of that story is that the viscosity of all the various lubricants in your vehicle has a major effect on your fuel mileage.  I suspect that a significant portion of your commute is devoted to getting them up to proper operating temperature.

rmw

In my area, school starts for the kids in the fall, and traffic during the morning commute is much greater (Moms & Dads dropping off kids, school buses grinding slowly along surface streets, fewer working people gone on vacation, etc.).  Thicker traffic means slower speeds (takes me an extra 10 minutes to get to work) and lower mileage.

It may not be scientific, but for tank after tank, over a 5,000 mile change interval, my F150 got high 18's on conventional 5W-30 and then low 19's on synthetic 5W-30; about .4 mpg difference.  I think over 5,000 miles, any conscious or sub-concious attempts to feather the pedal and increase mileage were overcome by my normal driving habits.

Blacksmith

Generally, a hot climate at high altitude = more mpg, cooler at sea level = less mpg.

Your tires probably dropped a couple of psi as well.  I was reminded of this when rotating the tires on one of my own vehicles this afternoon.

Norm

Another reason for increased fuel consumption at lower temperature is increased resistance on the air intake.  The increase in engine efficiency with increase in air density only helps when accelerating. i.e.,  at full throttle.  The vast majority of the time the throttle plate is partially closed, e.g, with the engine making perhaps 2200 revolutions per mile at highway speed.  So, for the same power, with the air being 9% denser, the throttle plate must be restricted about 9% further.  Ergo, more intake friction.  

The 9% comes from the change in absolute T:
A temperature decrease from 87 to 42^oF (303.7 to 278.7 Kelvin) increases the air density by a factor of 303.7/278.7, or 1.0897 times that at 87^oF.  

A diesel engine shouldn't suffer from this particular effect.  Can any diesel drivers comment?

Intake friction?  NO. Insignificant effect on part throttle econ.  Pumping loss is reduced as a result of lower density due to less baro pressure and higher temps.  Less delta P across the piston for the same bmep = less parasitic loss.  Yeah there are other factors such as reduced drag on the car at high altitude and high temp as well.

Well, FWIW, it's been in the upper 50's / low 60's the past few days, and NO RAIN.  31.5 mpg yesterday on the way home.  I'm convinced that first off, my driving style has the biggest impact, with believe it or not, rain being second for the reasons rmw stated.

Just want to say thanks for all the input.  Now if I could just wake up early, I'd be getting good mileage on the way TO work as well.