Winter/Summer Fuel Economy

[CR94]

Originally Posted by RayCJ
... As for air density, if memory serves, for every 10 degrees (Fahrenheit) drop, density increase approximately 5%.
...
Nearer 2%. A more accurate and even simpler rule is that air density (hence drag) is inversely proportional to the (absolute) temperature, assuming reasonably constant barometric pressure.

 

[bmwpowere36m3]

Goes from 16-18 MPG summer to 12-14 winter....

'95 E320, DD, mostly short trips and more idling winter

 

[barryh]

My car does get worse MPG in the winter but it's a relatively small effect perhaps between 5% and 10% That much is inevitable with longer warm up times, lower calorific value of fuel and the increase in air density. With my bike which is air cooled, the winter effect is more pronounced and predictable. The bike suffers from the same factors although the aero effect will be much bigger and being air cooled it really never reaches the same operating temperature as it does in the summer.

If you ignore the rising trend the gragh shows a very consistent winter reduction in mpg over 11 years of 15 -20%.

 

[bbhero]

Originally Posted by 69GTX
The denser ambient air from dropping around 40-60 deg F between summer to winter is responsible for an additional 10-15% of wind resistance on the vehicle. That's probably the largest factor in mpg. If you run or bike outdoors all year round, you know the effects.




^^^^^^^^^

That makes a ton of sense... At the 1000 Mb to 850 Mb level in the atmosphere thickness values dictate whether that level is below freezing 0°C. Lower than 130 decameters is less than 0°C. Where less dense air above that marks air temps above 32°F.

 

[CR94]

Originally Posted by Burt
I always thought that 50-60 deg f was temperature that results in best mpg. Its halfway between -20 F and 120 F, so the sweet spot.
No, hotter is nearly always better for mpg, or maybe realistically the hottest you're willing to tolerate without air conditioning.

 

[RayCJ]

Originally Posted by CR94
Originally Posted by Burt
I always thought that 50-60 deg f was temperature that results in best mpg. Its halfway between -20 F and 120 F, so the sweet spot.
No, hotter is nearly always better for mpg, or maybe realistically the hottest you're willing to tolerate without air conditioning.



I agree... Best mileage in all my cars happens when outside temps are somewhere between 70 and 90. When temperatures are on the high side, I resist using AC within practical limitations. When possible and when the car is in motion, I just crack open a couple windows by an inch or so. This gives decent air circulation when you're moving but does not create excessive wind drag. Driving with windows rolled-down creates massive wind drag which can reduce economy more than the AC unit.

 

[Uphill_Both_Ways]

In summer I get an indicated 5.6 to 6 litres per 100 kilometres on a much-frequented 70 km/h (42 mph) road with the air conditioning on. (Six is 47 mpg imperial, 39 mpg U.S. 100 kilometres is 62 miles.)

When the outside temperature drops below 4°C or 5°C (forget any wind-chill temperature; it means nothing to inanimate objects), that drops to 6 to 6.2 litres per 100 kilometres (the air conditioner stops running at 4°C).

Colder temperatures are yet to occur this year, but when it hits -25°C and colder, it drops farther, but always with the engine up to operating temperature.

I reset the average readout (the metric instant readout doesn't include the decimal point) at cruising speed to eliminate the idling and city-driving average, which could be anything from seven to 9-plus, and sometimes down to an abysmal 15-plus litres per 100 kilometres in winter rush hour, ~13 in summer.

 

[xBa380]

My 2018 Hybrid SE Camry... Got 44mpg before it got cooler out, now around 38.

 

[CT Rob]

17 Tundra crewmax 4x4 5.7, I go from 18mpg down to 15.3-15.7 in winter with our winter gas here in CT.Around April or so it will shoot right back up .That's hand calculated .I dont warm it up either ,just get in and go