Changes in MPG related to temperature?

[MillCreek]

Since I buy new cars so infrequently, it is always a joy to discover what bells and whistles are being put on cars nowadays.  My 2011 Kia Soul, which replaced my 1986 Mazda pickup, has a real-time MPG calculator.  I like to see how high I can get the MPG in a given driving condition.

My main route to and from work is along a scenic rural road with lots of hills and twisties.  I drive the same route to and from work.  I reset the MPG calculator at the start of the morning and afternoon trip.  I have noticed that in the morning, with cooler temperatures, my real-time MPG is consistently higher than my afternoon trip with higher temperatures. 

The temperature difference is the only variable; well, that and it is dark in the morning and light in the afternoon, but I don't think that makes a difference.  I did some Googling and noticed that most of the time, people report increases with MPG with increases in temperature; the opposite of my findings.

I am wondering if temperature-related changes in fuel or air density may explain this.  Or perhaps tire rolling resistance related to temperature.

[Tallpine]

Yeah, the tires would be harder in cold.

But the cold air would be denser thus more drag.

Maybe it's downhill to work and uphill back home ...?   

[RocketMan]

Cold air is denser, hence more air with more O₂ in the combustion chamber during the power stroke, maybe?
Also, I've read that it is a higher engine temperature that helps mileage, not ambient temperature.
I wish I had that kind of doohicky in my truck.  I do have a shift indicator, but that's all.  I wonder if I could gen something up using a microcontroller and a few aftermarket sensors?

[Tallpine]

Cold air is denser, hence more air with more O₂ in the combustion chamber during the power stroke, maybe?

Yeah - most vehicles now have ducting to suck in cooler outside air to feed the engine, as opposed to engine compartment air.

[RocketMan]

Wait a sec, it's a Kia.  Doesn't Kia mean "lots of squirrels" or something similar in Korean? 

[Ben]

I have the MPG meter (aka lie-O-meter     ) in my diesel truck. I have a relatively flat ~250 mile (one way) trip I take a couple of times a month at lower elevation (a couple of passes up to 2500', but short duration). I average 19mpg on it. I can't remember for sure, but I think I may get about 1mpg higher during the winter than during the summer (when it may be in the 90's along my route).

A few times a year I take it to the Mammoth area of the Eastern Sierra to go fishing. At the high altitude (7000 - 10000') I average 22mpg. The ambient temp is generally cooler than where I live, but I think altitude may have something to do with improved MPG as well.

[RocketMan]

Does your truck have a turbo, Ben?  If it does, I wonder how that factors in with the mileage vs. altitude thing.

[Doggy Daddy]

Altitude difference is the first thing I'd check.  Hover your mouse over each destination in Google Earth, and it'll show you the altitude.

DD

[MillCreek]

Wait a sec, it's a Kia.  Doesn't Kia mean "lots of squirrels" or something similar in Korean? 

I believer that for this model, it means 'rapping hamsters'.

[Hawkmoon]

Cold air is denser, hence more air with more O₂ in the combustion chamber during the power stroke, maybe?

That used to be true with carburetors. It's not true with electonic fuel injection, because the air-fuel mix is controlled by the oxygen sensor in the exhaust manifold or pipe, and it optimizes the injection to maintain the most efficient air-fuel ratio regardless of engine speed, car speed, load ... or air temperature (air density)

[Ben]

Does your truck have a turbo, Ben?  If it does, I wonder how that factors in with the mileage vs. altitude thing.

It does. I wouldn't be surprised if that has an effect, though I don't know much about turbos other than they make truck go now.

[zxcvbob]

If you live way far north, they change the gasoline mixture in the winter -- probably add in a lot more butane.  My mileage in my truck drops from about 17 in the fall to about 10 or 11 in the winter.  I recently drove from Minnesota to Alabama in a Ford Focus, and the mileage jumped about 10% when I switched from MN gas to southern Missouri gas.  And the change coincided exactly with filling up my almost-empty gas tank in MO.

[griz]

With the hills and twisties you describe, I would suspect a difference in the route.  Even if both ends are at the same elevation, if the drive has a lot of downhill braking in one direction, but a throttle free downhill coast going the other way, that would account for it.

I'm glad to see that I"m not the only one that likes to play with that gizmo.  I reset mine at Cripple Creek Colorado and drove toward Colorado Springs, about 4000 feet lower in elevation.  Halfway there I had maxed out the thing at 99 MPG.

[sanglant]

don't forget oxygenated fuel. and a turbo should reduce the effect of alt. changes. oh, and fuel is denser when cold.(but it shouldn't be a noticeable change)

oh, and try accelerating faster. and shifting at lower engine speeds.

[charby]

I have a supercharged pickup and my mpg drops about 10% in the winter. I actually seem to get my best MPG in the spring and fall.

[HankB]

If you live way far north, they change the gasoline mixture in the winter -- probably add in a lot more butane.  My mileage in my truck drops from about 17 in the fall to about 10 or 11 in the winter.  I recently drove from Minnesota to Alabama in a Ford Focus, and the mileage jumped about 10% when I switched from MN gas to southern Missouri gas.  And the change coincided exactly with filling up my almost-empty gas tank in MO.

When I lived in MN, Archer-Daniels Midland, a major producer of fuel alcohol, bribed convinced via lobbying the MN legislature that diluting gas with ethanol was a good idea, so they mandated ethanol-diluted gas in the Twin Cities area during the winter. In comparing notes with colleagues, this meant at least a 10% loss of fuel economy, rather than the 3% the lying b@$t@rdS fuel experts claimed.

I used to drive frequently between the Twin Cities and New Ulm (220 miles round trip) and found that during the winter months, New Ulm gas yielded about 10%-12% better fuel economy than Twin Cities gas. No difference during the summer . . .

[French G.]

That used to be true with carburetors. It's not true with electonic fuel injection, because the air-fuel mix is controlled by the oxygen sensor in the exhaust manifold or pipe, and it optimizes the injection to maintain the most efficient air-fuel ratio regardless of engine speed, car speed, load ... or air temperature (air density)

True, but if the 'puter is trying to maintain optimum fuel/air ratio you will make more power in the cooler air. A few extra HP can keep the driver's foot a little more out of the throttle because they perceive that they are getting where they want to be fast enough.

The difference could be as simple as subconscious driver attitude. Maybe you mash the gas a little more getting home. Could be same top speed if you are allergic to  but just quicker accelerations. In my case I'm sure it was reversed when I commuted, I don't think I ever woke up and thought I had plenty of time to get to work.

[Ron]

My gas mileage goes down in the winter.

I've been driving the same car since '94 so I have many years of observation of this phenomenon.

[zahc]

That used to be true with carburetors. It's not true with electonic fuel injection

It's still true to some extent because the engine's volumetric efficiency is affected.

My dad's semi trucks always got worse mileage in the winter time. He attributed it to denser air causing more wind resistance.

I suspect the OP has an altitude difference between his going vs coming routes.


On the topic of instant fuel mileage calculators, my Buick has one. It also has a gizmo that tells you how many miles left you have on your current tank of fuel. We typically stay in City for the most part, where the Buick gets about 16mpg. One time we went on a short trip up north, and the range estimator said our remaining range was 270 miles when we left and it steadily increased the whole time we were on our trip until it said 340 miles when we got to our destination. We should go on long trips more often; it's like getting free gas!

[MillCreek]

I suspect the OP has an altitude difference between his going vs coming routes.

I would not think so, since I am driving exactly the same route in the morning as I do in the afternoon, just in reverse.  But this does make me think about weather influences; specifically barometric pressure, as a proxy for altitude.  Hmmm.

[280plus]

I believer that for this model, it means 'rapping hamsters'.

You can get wit dis or you can get wit dat...

I kind of like the toaster myself.

[Tallpine]

I would not think so, since I am driving exactly the same route in the morning as I do in the afternoon, just in reverse.  But this does make me think about weather influences; specifically barometric pressure, as a proxy for altitude.  Hmmm.

Is it downhill both ways ?   

[MillCreek]

Is it downhill both ways ?   

If it was, I would be riding one of my bicycles, with a big smile on my face.

[280plus]

Just off the top o my head I do know that fuel expands and contracts with temp so possibly less fuel/more energy per unit when cold and vice versa when warm?

[Tallpine]

Just off the top o my head I do know that fuel expands and contracts with temp so possibly less fuel/more energy per unit when cold and vice versa when warm?

There would be a several hundred gallon shortage when a tanker would haul a load of fuel from Grand Junction to Gunnison, Colorado