Cold weather on auto transmission and MPG

[Dunks001]

I was traveling in the north east recently and saw reduced MPG and weird shift points. Going down the highway the engine was running at higher RPMs. I kept catching her in 5th gear. Got 14 mpg on the tank when I typically get over 16. Temps were in the single digits. The truck is used to warmer texas temps. The engine and transmission were at proper running temps. Maybe it was winter blend fuel, driving style and/or head winds? The transmission would stay in 7th after manually shifting.
Going in for an oil change tomorrow and thought to see if there is anything that needs to be checked.
21 Max Tow, 37's, 40k miles

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[dcmdon]

Air is thicker when it's cold. Much thicker.

In NH in the winter I get 15ish these days. In the summer, I get 18ish.

Because the air is thicker it takes more power for the engine to push your truck at a given speed. At highway speeds, aerodynamic drag makes up the majority of the drag on the truck, so the hit is biggest on the highway.

The transmission responds based on how much power is required. If more power is required to accelerate at a given rate or cruise at a given speed, then it's going to hold lower gears longer. Just like if you were pulling a trailer.

So in summary, what you are describing is perfectly normal.

From an earlier post I made about a simolar situation.


Cold air does increase horsepower.

Cold air is denser. So there is more mass of oxygen (more oxygen) in a given volume of air. So if your throttle is open a certain amount, in the winter it will let more air in and can make more hp.

But in order to make more hp, your fuel injection meters in more fuel. So for a given throttle opening, you get more air AND more fuel. More power. With more fuel burn.

But that doesn't actually work against you because you need X hp to maintain a certain speed. So you compensate by opening the throttle less.

In the end the throttle is open less to make the same hp when it's cold. I hope this makes sense.

So again, from an engine efficiency perspective cold air is irrelevant.

However what kills you is the actually thickness of the air that the engine needs to push your truck through. Think of air as a fluid. At higher temps it's easier to push your truck through that fluid because its less viscous. That means that at highway speeds, where most of you fuel burn is made up overcoming aerodynamic drag, you will burn less fuel when its warm.

I never realized how significant this thickening of the air in cold weather was until I started flying. The wings on an airplane make lift by being moved through the air. At lower temps a plane can fly at a lower speed because the air is thicker.

I'm looking at the takeoff performance chart of an airplane I used to fly.

At 32 deg the take off roll is 795 ft.
At 85 deg the take off roll is 1010 ft.

Drag and lift are both linearly impacted by the viscosity of the air. So 32 deg air 27% thicker than 85 deg air. (1010/795 = 1.27)

So in short, at 32 deg, the aerodynamic drag your jeep has to overcome is 27% more than it is at 85 deg.

 

[Dunks001]

Amazing. Thank you.

 

[ShadowsPapa]

I was traveling in the north east recently and saw reduced MPG and weird shift points. Going down the highway the engine was running at higher RPMs. I kept catching her in 5th gear. Got 14 mpg on the tank when I typically get over 16. Temps were in the single digits. The truck is used to warmer texas temps. The engine and transmission were at proper running temps. Maybe it was winter blend fuel, driving style and/or head winds? The transmission would stay in 7th after manually shifting.
Going in for an oil change tomorrow and thought to see if there is anything that needs to be checked.
21 Max Tow, 37's, 40k miles

Mine hardly ever sees 8th, very rare, and 7th is not common. A lot more time in 5th and 6th with the single digits and below 0 temps.

Yes, a ton more drag, cold rubber tires resist rolling, denser air.
Cold air intake only does so much -as said, more power, but honestly, it's only real noticeable if you are comparing time slips at the track. Open the hood to cool the engine bay while in the staging lanes and you may drop a tenth off your time. It's not nearly enough to offset the negatives of these trucks.

One thing I've noticed is that where a 5 mph wind would have xx impact on my truck in the summer, that 5 mph wind may as well be 10 or more at 0 degrees.

 

[timwiller]

Nice explanation.
I miss flying...

 

[Dunks001]

One thing I've noticed is that where a 5 mph wind would have xx impact on my truck in the summer, that 5 mph wind may as well be 10 or more at 0 degrees.

Yup. @dcmdon how bad would that drag be if we drop the temps 10 degrees (22F) and add a 5 mph head wind.

 

[dcmdon]

Re temp change, I don't know off the top of my head. The charts don't go any colder than 32 deg.

But let's play wit that 5 mph headwind.

What we're talking about is the difference in aerodynamic drag caused by the head wind.

If we assume 70 mph driving speed and a 5 mph head wind, then what we are finding is the difference in aerodynamic drag between 75 mph and 70 mph.

Aero drag increases as a square function of speed. So.

(75/70)^2 = 1.1479

So a 5 mph headwind while driving 70 mph increases aero drag by 14.79%.

I hope this helps.

 

[Dunks001]

i love smart people! Thx.
Surprised the charts don’t go lower than freezing.

 

[Hristo]

Good explanations. OP I saw the same sort of results on a long trip I recently took where temps ranged from -5C to -43C. The fuel economy took an absolute dump in the colder weather.

 

[Blade1668]

I hope this helps.

Dang good info, gotta love having so many diverse backgrounds on here. Tons of information on about everything if asked.

Side note:
My drive time took a bigger hit than the MPG's about 4 plus more hours... still a great drive after getting away from many out on roads.

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