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Question for people who know something about cars
Old 12-16-2016, 08:55 PM   #1
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Question for people who know something about cars

Twice this week, DW and I had to do a 5 hour round trip road trip, for reasons that are outside the purview of this conversation. Our new car has one of those fancy computer displays that keeps track of your mileage for you. On the first trip we got 27.1 mpg. But on the second trip (today) we only got 25.6 mpg. Both times I drove (I love to drive). I drove the same car, the same route, the same 64 mph cruise control setting (for the most part), with gas from the same gas station, etc. The only difference that I can come up with is that the temperature was in the low 50s the first day, and in the low 20s today. Does the outside temperature effect the mileage that much? No big deal, really, but I'm always curious about numbers.
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Old 12-16-2016, 09:07 PM   #2
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The lower ambient temperature certainly can cause a poorer engine efficiency.

Another big factor is the wind. If you had a 10 mph wind and a ground speed of 64 mph, a head or tail wind would mean an airspeed of 74 mph or 54 mph in the extreme cases. As the wind drag increases with the square of airspeed, that means a ratio of (74/54)^2 = 1.88X in wind resistance.

PS. Our 1995 minivan has that mpg display. I always reset the display when we set out for a round trip between our 2 homes. The mpg for that 290-mi trip varies quite a bit each time with who knows what.
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Old 12-16-2016, 09:08 PM   #3
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All my years commuting I always got better spring and fall milage and lower summer and winter milage, generally about 2mpg as you got.
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Old 12-16-2016, 09:18 PM   #4
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Another factor is that cold air is denser than warm air, hence causes more wind drag.

I have made many trips with my RV, and try to maintain a constant speed for the best fuel economy. I invariably observe better gas mileages at high elevation vs at sea level. I believe it is because of less air drag with thinner air in the high plains. At 6500 ft, the air is about 15% less dense than at sea level.

Basic aerodynamic equation:

F = (1/2) x (rho) x (V^2) x Cd x A

where:

F = drag force
rho = air density
V = airspeed
Cd = drag coefficient
A = frontal area
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Old 12-16-2016, 09:22 PM   #5
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Originally Posted by NW-Bound View Post
Another big factor is the wind. If you had a 10 mph wind and a ground speed of 64 mph, a head or tail wind would mean an airspeed of 74 mph or 54 mph in the extreme cases. As the wind drag increases with the square of airspeed, that means a ratio of (74/54)^2 = 1.88X in wind resistance.
+1

Wind can make a huge difference in fuel mileage.
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Old 12-16-2016, 09:31 PM   #6
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Pilots know this very well.
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Old 12-16-2016, 09:59 PM   #7
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I think it is a combination of things. The higher air density, changes in tire pressures, changes in gas formulations, longer times to get all the lubricants up to temperature. I see the same thing with my vehicles. Between the best days of spring and fall and the coldest/hottest days of summer I can see a 5-7 mpg difference in my Subaru.
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Old 12-16-2016, 11:38 PM   #8
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Lubricants are much thicker when it's cold. Anything mechanical that relies on grease, oil, or any lube is going to have more drag due to the thick, cold lubes.

I imagine your drivelines, differentials, steering controls, etc all had more drag.

Also tires;
Air pressure is partly based on air temperature. Colder temps and the tire pressure is less. Less tire pressure and your mileage could drop a couple MPH. If you didn't check your tire air pressures and set them for the conditions, then that's probably it.
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Old 12-17-2016, 01:55 AM   #9
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Wind, air density (temp), up or down a hill.

I've seen similar results in our Honda Odyssey.
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Old 12-17-2016, 04:20 AM   #10
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Quote:
Originally Posted by harley View Post
On the first trip we got 27.1 mpg. But on the second trip (today) we only got 25.6 mpg. Both times I drove (I love to drive). I drove the same car, the same route, the same 64 mph cruise control setting (for the most part), with gas from the same gas station, etc. The only difference that I can come up with is that the temperature was in the low 50s the first day, and in the low 20s today. Does the outside temperature effect the mileage that much?
I just drove from Chicago to S Florida and noticed about the same difference in mileage. My guess was the car heater caused some of that difference. There's no doubt that mileage improved as we headed south.
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Old 12-17-2016, 04:30 AM   #11
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While there is some variation, gasoline basically holds a constant amount of energy. When it is colder, more of that energy is lost to the environment so there is less to push the car. My mileage is steadily 3-4 mpg lower in winter.
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Old 12-17-2016, 04:45 AM   #12
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It also depends on other traffic. If you are ideally drafting the car ahead of you, you mileage will increase nicely. If you are in the wrong part of the turbulences...your mileage will suck. I'm not recommending tailgating for better mileage, but you could see that.
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Old 12-17-2016, 06:01 AM   #13
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In cold weather, your choke will run a rich mixture for longer after you start out, because it takes the engine longer to warm up. That will cut into your mpg.

(Yes, I know fuel injected cars don't technically have a choke, but they do change the fuel/air ratio by adjusting the opening time of the injectors and they do run richer when the engine is cold).
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Old 12-17-2016, 06:29 AM   #14
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Originally Posted by REWahoo View Post
+1

Wind can make a huge difference in fuel mileage.
I get between 33 and 40 with my Honda civic depending on the winds.

If you put larger tires on the back wheels, it will be like going downhill, and you will get better mileage.
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Old 12-17-2016, 07:19 AM   #15
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Gasoline formulas in the winter in colder regions have more volatile components to make cold starts easier. The 20 different formulas change in September and May and the biggest reason for price changes. Lower volatile blends were created to lowers emissions, and are affected by temperature, elevation and population.

While the colder air is more dense and increases drag, it also increases the oxygen available and makes then engine burn more efficient. Many cars are designed to get the coolest air available to your cars intake.

I would say the biggest culprit in your mileage is your tires. Proper inflated tires allow the car to roll easier, and temperature changes greatly affect this. A properly inflated tire in the Midwest, driven to Florida, would result in a over inflated tire, upon arrival. Also, inclement weather such as rain or snow decreases traction and therefore some slippage occurs.

And of course, YMMV.
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Old 12-17-2016, 07:38 AM   #16
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If you put larger tires on the back wheels, it will be like going downhill, and you will get better mileage.
Only drive west in the morning and east in the evening so the solar wind will help push you along and improve your mileage!
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Old 12-17-2016, 07:38 AM   #17
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Wind is a major factor. It doesn't even have to be head-on to make a measurable difference. Side winds can also affect MPG. Different electrical requirements can also be measurable in today's high MPG cars. Headlights, heater blower and certainly defrosters are some. Elevation changes are another. The gas pumps say "may contain up to" 10 % Ethanol, but what is it exactly? Ethanol free gas gets me about 7% better MP, but I can't buy it within 50 miles of my home. My Hyundai Elantra varies all over the map from 33 to well over 40 much like Senator's Civic.
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Old 12-17-2016, 08:38 AM   #18
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I get between 33 and 40 with my Honda civic depending on the winds.

If you put larger tires on the back wheels, it will be like going downhill, and you will get better mileage.
Well who can afford to do that? Not me. So instead I took the wheels off my kid's bike, and put them in the front.
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Old 12-17-2016, 09:33 AM   #19
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The OP drove the same route, filled up with the same gas, so one can rule out terrain and the difference in gasoline blends, though those certainly have a big effect.

About pitching the car down with larger rear wheels, that is a common misconception. When on a slope, the car rolls downhill. However, it does not roll forward by itself if one raises its rear (you would have discovered perpetual motion). If that causes any change in gas mileage, it may be explainable by the change in air drag.

If the effect were real, one would not be able to ride this bicycle forward.

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Old 12-17-2016, 09:35 AM   #20
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Originally Posted by NW-Bound View Post
Another factor is that cold air is denser than warm air, hence causes more wind drag.

I have made many trips with my RV, and try to maintain a constant speed for the best fuel economy. I invariably observe better gas mileages at high elevation vs at sea level. I believe it is because of less air drag with thinner air in the high plains. At 6500 ft, the air is about 15% less dense than at sea level.

Basic aerodynamic equation:

F = (1/2) x (rho) x (V^2) x Cd x A

where:

F = drag force
rho = air density
V = airspeed
Cd = drag coefficient
A = frontal area
Didn't consider air density. Found this:

https://www.gribble.org/cycling/air_density.html

and the air @ 20F is about 6% more dense than at 50F, so 6% higher drag per your formula, and drag is a fairly high component at highway speeds (you still have engine losses).

Quote:
Originally Posted by MichaelB View Post
I just drove from Chicago to S Florida and noticed about the same difference in mileage. My guess was the car heater caused some of that difference. There's no doubt that mileage improved as we headed south.
Heaters in non-EV cars don't really require any energy, they are using the waste energy from the engine. And in warmer weather, the radiator fan might kick on more, and may use more electrical energy than the heater fan was in cold weather.

Quote:
Originally Posted by Winemaker View Post
Gasoline formulas in the winter in colder regions have more volatile components to make cold starts easier. The 20 different formulas change in September and May and the biggest reason for price changes. Lower volatile blends were created to lowers emissions, and are affected by temperature, elevation and population.

While the colder air is more dense and increases drag, it also increases the oxygen available and makes then engine burn more efficient. Many cars are designed to get the coolest air available to your cars intake.

I would say the biggest culprit in your mileage is your tires. Proper inflated tires allow the car to roll easier, and temperature changes greatly affect this. A properly inflated tire in the Midwest, driven to Florida, would result in a over inflated tire, upon arrival. Also, inclement weather such as rain or snow decreases traction and therefore some slippage occurs.

And of course, YMMV.
So now we have to consider the engine eff% improving while air drag makes things worse - this gets complicated! Lots of effects mentioned in this thread - air pressure in tires could be significant.

I doubt that the thicker lubricants and 'choke' in the cold would matter much on a long trip, these would be warmed up and the 'choke' off once you are driving a few miles.

-ERD50
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