Cold effects on ICE only?

My data indicates that it takes about 30 minutes for the transaxle to fully warm-up. Also, there are temperature effects on tires that can increase rolling resistance and the air is denser for more drag.

Bob Wilson

 

Also: cold -> squishy tires -> lower MPGs.

 

Watch what happens to your MPG on a cold, but very moist winter day. It's climbs up quite a bit.

Humidity contributes to compression. The water vapor expands allows the engine to work less for the same amount of power.

Winter air is usually bone dry. So among all the other factors working against efficiency, you've got that too.
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If you keep the pressure on your tires topped up, they won't get squishy. On the contrary, the rubber gets less flexible when it's cold.

 

Yes.

Lower temerature causes:
1. more aerodynamic drag
2. more pumping losses in the ICE air intake system (unless it is compensated by an intake air heating system)
3. more friction in the ICE from the colder oil
4. more friction in the gearbox from the colder oil
5. more friction in the wheel bearings from the colder grease
6. more tire rolling resistance from lower tire pressure (unless you adjust air pressure every time it gets colder)
7. more tire rolling resistance from less elastic rubber.

It all adds up.

I'm surprised nobody mentioned wind yet. A gentle breeze that you otherwise wouldn't even notice can make a significant difference at freeway speeds. Here is some really interesting information on this: Wind is a BIG DEAL

 

Your gasoline engine has only two fuels - gasoline and air. Since air temperature changes, the temperature of your fuel mixture in the engine cylinder is also changing. Air temperature, humidity, even altitude can all affect your mileage.

 

Colder air is more dense, so it can increase milage at lower speeds due to a a bit more power generated in the engine... but that may be offset by increased air resistance...

 

No, because inflated tires get almost all of their stiffness from internal air pressure, which increases as temperature increases. It is true that the tire material gets softer as the temp goes up but the effect of this is negligible.

 

Recently I drove 800 miles through 15F to 35F, dry air and was able to monitor my mileage. Instead of the usual 53 MPG @ 65 miles per hour, I was running 49 MPG or worse at temperatures below freezing. Thanks to Ken@Japan who provided the air densities at different temperatures, I was able to put this spreadsheet together:

TempF TempC density_kg/m3 %_density MPG
15 -9.4 1.34 111.7% 47.5
32 0 1.29 107.5% 49.3
68 20 1.20 100.0% 53.0
85 29 1.17 97.5% 54.4

There was good agreement with the air density changes and MFD displayed mileage. The MFD has a horizontal scale line at 50 MPG so it was easy to see the pattern. Below freezing, I couldn't bust 50 MPG except by driving slower. Above freezing, 65 mph returned 50 mpg or above. I'm driving an NHW11 03 Prius but the physics works for all vehicles.

There may be other effects but not that I could detect on this trip.

Bob Wilson

 

I also find significantly lower MPG in cold weather but have noticed that the MPGs are MUCH lower when I have my heater on high and the windows down. (for cigar smoking on a late night drive between Vegas and SoCal).

I'm pretty sure that the traction battery supplies at least some, if not a lot, of power to the heater.

 

I am pretty sure that a strong headwind is the culprit here. I drive that route on Hwy 99 or I-5 frequently. There is a normal 10 to 20 MPH northbound wind coming through the grapevine and up the valley.

In fact, I usually get better than normal gas mileage heading Northbound and, of course, worse than average, southbound.

At 70 MPH, a 10 MPH headwind is almost the same as you driving at 80 MPH in terms of fuel consumption.

Heading south, once you get over the grapevine into LA, then the wind issue goes away and its warmer too.

 

Most of the heat comes from the engine water cooling system - there are a couple of small electric heaters, but they won't have any significant effect on your mileage.

I think in your case, it's the open windows that hurt your gas mileage.

 

That only applies when the throttle plate is wide open (floored... in layman's terms.) As long as you are not floored, then the density of the air will not be a factor in the engine efficiency or horsepower generated (within a normal temperature range as discussed here.... not super extremes).