I've seen this a few times where people claim that cold weather will produce more drag on a vehicle just the cold dense air. So just for the fun of it I went and did the calculation on just the air drag part of it. Not factoring in other loses like fluid thickening...

Here is the formulas I used

Drag

http://www.thefintels.com/aer/dragcalc.htm

Air Density

http://www.denysschen.com/catalogue/density.aspx

Dodge Ram Specs including Drag Coefficient

http://www.pickuptrucks.com/html/ram_specs.html

So using local information and building a test bed on this.

Vehicle - 2nd Generation Dodge Ram 2500 truck

Test #1 Winter (Column C)

Stats

[*]+10*F Temperature

[*]90% Humidity

[*]2,800 ft Elevation

[*]45 MPH (Road conditions locally)

Test #2 Summer (Column B)

Stats

[*]+100*F Temperature

[*]10% Humidity

[*]2,800 ft Elevation

[*]65 MPH

Test #3 Comparing both using both summer and winter conditions. (Columns E & F)

Which this shows roughly 4.659 MPH difference between winter and summer condition will be nearly equal in drag.

Just for fun compare 55 and 65 MPH...

Thats pretty significant actually. Thats at 55 where there isn't even any real speed. Do the calculations at 70mph.

Well at 70 MPH it sure changes but when winter time is around we typically don't have highway conditions to drive 70 MPH. Let be realistic here too can 70 MPH be done on snow covered roads or icy conditions safely? Not to mention the only place I know in the entire state of Idaho that is 75 MPH are the interstates and they are not around here. But cold weather won't degrade city driving or rural driving very much. I think it going to be mostly fluid drag. (Gear lubes etc.)Everything from 55 MPH changes very drastic... Most all site I've been across all state the same thing that regardless drag is exponential and climbs a steep curve after 55 MPH.

At low speeds the drag doesn't change much, but the fluid is colder and doesn't warm up nearly as much since the power requirement is low so your back to sucking more fuel. Cold weather does have a huge effect in city/rural driving as well, just different causes. In regards to 65-75 mph, just because it's cold doesn't mean there is snow and ice on the roads, and not everyone who sees a difference in mileage lives where you do. Snow covered roads are a hit on mileage in themselves as the drag from the snow is much greater than from a bare improved surface. It is very possible to do 70 on a bone dry road a 0°F or colder. That drag coefficient of .44 seems really low considering my bullets for hunting are .444, and they appear a LOT more aerodynamic than your truck. Drag in general increases the colder it gets, some from air density, some from fluid, some from surface condition. Don't forget that the same increase in density has an effect on the frontal area and the weight of the vehicle. The weight increase is minimal, but lots of little things add up.

I ask ISX to rework the sheet a bit more and cover a wider range but even when he asked about the the 70 MPH its only a 6 MPH difference from 100*F to 10*F. so if you just slow down 6.xxx MPH its the same drag. But once again I ask him and his talents of re-working the formula to possibly include a graph. Still in all I provided all information of the drag and everything including formulas.

It is very possible to do 70 on a bone dry road a 0°F or colder.

Still not a wise thing to do and this is what is creates so many slide off around here. Is around the next corner might bet completely a sheet of ice and it too late to slow down or that one bridge might be completely froze over and go gliding over into the river. I seen the my buddy with the tow truck pulling that poor sole out of the river lucky man. I've seen road conditions so slick that a person physically can't stand up only last less than 100 yards and no ice present before or after or even warning of it.

I ask ISX to rework the sheet a bit more and cover a wider range but even when he asked about the the 70 MPH its only a 6 MPH difference from 100*F to 10*F. so if you just slow down 6.xxx MPH its the same drag. But once again I ask him and his talents of re-working the formula to possibly include a graph. Still in all I provided all information of the drag and everything including formulas.

Good info, that is a bit of difference. 6 mph is close to From Cummins

Ambient Temperature Air becomes more dense as temperatures drop, which increases air resistance. For every 10° F drop in temperature, aerodynamic drag increases by 2%. Thus, fuel efficiency will drop by 1%. Overall, fuel economy tends to be higher in the summer than the winter. According to North American Truckload Fleet Data, driving in the summer increases fuel mileage by 8 to 12% over driving in the winter months.

Additionally

Fuel Blends While blended fuels provide better startability and protection against fuel gelling than standard #2 diesel, fuel efficiency decreases. “Summer” fuel improves mileage up to 3% more than “winter” fuel.

That is blended fuel, not chemically winterized fuel. This shows that aerodynamic drag is greater than BTU changes. http://cumminsengines.com/uploads/docs/Secrets%20of%20Better%20Fuel%20Economy_whitepaper.pdf

Still not a wise thing to do and this is what is creates so many slide off around here. Is around the next corner might bet completely a sheet of ice and it too late to slow down or that one bridge might be completely froze over and go gliding over into the river. I seen the my buddy with the tow truck pulling that poor sole out of the river lucky man. I've seen road conditions so slick that a person physically can't stand up only last less than 100 yards and no ice present before or after or even warning of it.

It all depends on location, there are plenty of times where it is unwise and plenty of times where it is 100% safe and acceptable.