A few years ago I was traveling across WY in winter. Temps around 0*. I discovered that keeping all flaps closed on the winter cover gave me better fuel mileage; about 2mpg.

That made me wonder what is the magic temp for a diesel engine? If the intake charge is too cold, it won't fire as well.

A couple of weeks ago I drove down to Amarillo, TX. Traveling down I-25, 80mph, about 45* ambient air. Fuel mileage was the same with flaps open or closed.

Traveling across New Mexico, 70mph, about 30* ambient air. I got a 2mpg increase with the flaps closed.

While not as scientific as I'd like it to be, it is interesting. Seems to indicate that about 30-35* is the magic number.

 

Flaps? What flaps? I do know that winter fuel is lighter than summer fuel and in that light, winter mileage is noticeably less than with summer diesel simply because there is less BTUs in the fuel by volume, but not 2mpg, more like 1mpg. Wind resistance on the other hand makes a huge difference. These trucks are about as aerodynamic as an outhouse! Further, these trucks never see really cold air after warm up because of the turbo. Inducted air is typically 20 degrees C higher than ambient and more, hence the intercooler.

 

Strange, seems like flaps open would be more aerodynamic and better for mpgs. When I take my pizza box out in the spring I notice it gets better mileage, but that's also when the summer diesel starts showing up. People do swear by their Cold Air Intakes (CAI), even the home built dryer hose types seem to do well.

It's my understanding the 2013s have an active air intake management, pulling from either inside the air compartment or outside depending on the temperature. Whatever temperature that is should give you the answer you're looking for.

 

Flaps on the winter cover for the radiator.

All these numbers came from the same tank of fuel, so that really isn't a factor.

Whether you get better aero with the flaps open or closed is debatable. With them closed, it's a flat front, inside a small pocket. Maybe that creates some drag. With them open, it allows the air to flow out of that pocket, into the engine bay, and under the truck. Which is better? IDK.

But flaps open or closed at 80mph didn't seem to matter. So aero is probably not the issue.

People do swear by their CAI's. But there's no real data to show that there's an improvement on a stock truck. No independent dyno sheets or efficiency studies. It's all based on perceptions, feelings, and the ever popular Butt-Dyno. I'm a scientist and the ultimate skeptic. I need proof before I'l plop down $250.

I'll look into that active air management, thanx for the tip.