Hi Basjoos:

___The colder - denser air drag is negligible. All you have to do is drive a game gauge equipped automobile at 35 mph (torque converter locked up) in 0 - 30 degree temps vs. 50 mph + in 60 + degree temps to see this statement is way overrated. You can prove it to yourself in your own automobile. Run 50 mph + in mid-day (maybe 50 degrees near Chicago at that time of day) and stick your hand out the window. Feel the push? Do the same at mid-night running just 35 mph when it is 35 degrees or less. The push is quite a bit less.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

That's true. Density can make a big difference. When I was in Utah at ~10,000 feet, I routinely got 110 mpg during long drives. The thin air provided less resistance. Back in Maryland, it immediately dropped 10 mpg.

Hi All:

Why is the fuel economy of an automobile worse in the winter than in the summer?

___At 35 degrees F, I would only lose ~ 3.5% in fuel economy due to increased density due to the colder ambient temperatures according to the actual air density change. At 35 degrees in the real world, I achieve FE in the low 80’s vs. low 100’s at 70 degrees F. This is actual real world. Cold air density as an explanation for FE losses in colder temperatures is negligible given the real world 20% drop vs. air density’s predicted 3.5% or thereabouts from 70 down to 35 degrees F.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

All I know is that on my commute to work when I crest this one big hill (I-26 under the Blue Ridge Parkway) at 60mph, when the temp is at 80F I will have gained 10mph while freewheeling down this hill. But when the temp is -5F, I have to use engine power to maintain speed going down the hill, and if I freewheel the car at this low temp, I will drop in speed while going down this hill. This reduction in coasting ability has got to translate into a reduction in mileage somehow.

Another reason that the cold dense, dry winter air reduces mileage is that it holds more oxygen per unit volume, so that your normally asperated ICE can generate more power (and use more gas) on this denser air intake (which is why they put intercoolers on turbo/supercharged engines). With hotter, more humid summer air, oxygen molecules are fewer and farther between (and some of the air volume is taken up by water molecules) so the engine computer leans out the fuel mixture to compensate, giving you a reduction in fuel consumption (and in power output). Hot, thin, humid summer air has the same effect on engine performance as operating it at a higher altitude.

This whole air temperature/density/humidity interaction is what is known as "density altitude" to aircraft pilots, who use charts to calculate its effect on stall speed, indicated air speed, and engine power output. Aircraft get their best mileage at higher altitudes where the air is thinner (reduced drag) and where they can lean out their engine air/fuel mixture for reduced fuel burn rates.

Hi Basjoos:

___First off, you are talking about 100 + mph in an airplane where aerodynamic effects form varying densities actually are significant and we are not speaking of large temp/density/pressure swings in an Insight traveling at 55 mph or less between 35 and 70 degrees F. There is larger drag from your tires and viscous losses in colder temps vs. warmer then the loss in FE because of a change in density after the same 35 degrees F temperature drop. That real SOB however is the inefficiency of the ICE with that cold AIT and fuel charge.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net