The Deets

Cars don’t get lower mileage directly because it is cold outside. The engine doesn’t care and actually likes the cold air it is intaking as it is more dense and therefore richer in O2. On the other hand, the gasoline that you purchase has fewer btu’s in the winter due to spec changes that allow for butane blending in winter but not summer. Butane only has 4 carbons and therefore has less power. (similar to ethanol btu issues) Less power = lower gas mileage.

we rented a car last weekend for my wife to drive on a vacation and it ended up being a prius. she drove it maybe 500 highway miles and based on how much gas we put in, it averaged in the high 30’s mpg. it was killer cold, obviously.

tire pressure goes down a lot in the cold weather, which can have a big impact on mpg. people forget that sometimes.

Blame it on the butane, eh?

Color me informed.

Butane, eh? Okay, so the engine is solid in the cold, but there is more to running a car than pushing the pistons.

My car takes miles to loosen up the transmission and in today’s cold air my car was still not shifting smoothly at any time in my 18 mile commute. Heater is on high, the tires are cold, steering is stiff, etc. So while the engine is effective, but all the systems it is pushing are less efficient…so the engine will still work harder and gas mileage will decrease.

Then, I have a 2005 Prius in my 3rd winter driving it and I guarantee you my mileage suffers in the cold, about 20% less. So much so, I began to wonder if it was programmed by Toyota to do so as a way to keep its battery stronger longer. But now I just think it is natural due to two major factors–
1) the above transmission factor, aggravated by the fact a hybrid has a more complex transmission trying to blend battery/electric motor into the gas engine…and you can hear how hard it tries to shift in the cold; and
2) batteries suck in the cold and this is universal to all batteries as they all suffer in cold temps; they give weaker power, discharge easier, and take more time to recharge.

But…even in this cold, it still averages more than 35 mpg and thus still performs better than 99% of USA’s vehicles.

I’d blame the Prius mileage on the battery. If your transmission isn’t shifting properly, then you might lose some mileage–but that is your transmissions fault, not the cold (my transmission works fine in the cold).

Tires are a little flatter due to the cold==less efficient. I’ll give you that.
However–heat is nearly free (fans use very little energy compared to an AC compressor). The rest of the car warms up in less than 1 mile so that isn’t to blame.

I’d blame 85% on the Butane, 5% on low tires and 10% on your car not shifting properly.

I’m getting about the same on mine. As you say, it’s the winter.

I’d say the 85-5-10 factors sound right for an automatic transmission nonhybrid vehicle, but for my anecdotal Prius experience, I’d roll out the factors like this–
50% battery, 30% butane, 5% tires, 15% transmission

The butane is a subtle factor, invisible to the feel of driving, only apparent via more frequent trips to the gas station, so it is hard to judge without some scientific measurement.

But more obvious factors are the added strain on the gas engine because of how weak the battery is in the cold, it is almost impossible to go even a block solely on battery power; and then it is noticeable how hard the transmission works trying to blend the power sources effectively under even modest acceleration in extreme cold.

The Prius driver doesn’t need an engineering study to hear and feel those two factors involved in the cold. It’s that obvious. Still, I have never been stranded or been distressed driving it, and wouldn’t trade it for any other mainstream USA vehicle today.

Heat isn’t exactly free - in order for there to be heat the engine needs to be running. One of the tricks my Insight does (and I believe the Prius does) is that it will shut down the engine when the car comes to a stop. But, in the winter the car keeps the engine running even when I’m stopped so it is able to provide me with heat, shaving off a few MPGs.

Yup, Prius does the same. Heat and headlights (both of which are used more in winter), air conditioning in summer (which I rarely use), and I suppose the CD/radio too all cry for some power, and I too have noticed the gas engine shutting off in winter less as a result. Probably a 5% factor unless most of your driving is on city streets.

Butane haters!

BTW, the ALAMN’s leased E85 vehicles have been firing up just fine in the sub-zero mornings. Hope to get the new flex-fuel Impalla this year — I see a lot on the road already.

E85…oh yeah, and I hear one of the GOP proposals for reducing the national debt next term is to remove all the ag and big oil incentives hidden into ethanol production and thus pass along to the gas/ethanol guzzlers the costs involved in the full production price of ethanol…it’s not a tax, right?

Okay, I’m still laughing at my last post, but don’t want The Deets to get linked by Powerline without my clarifying statement here–
“My last posting above was full fabrication (we all know the GOP has no original thoughts involving ag or big oil and will perpetuate these subsidies until death do our country part).”

E85 should start up just fine–it has a very high RVP (vapor pressure), meaning that it vaporizes very easily.

Biodiesel on the other hand, clouds at a temp around 35 F–and cloud is bad when you are talking engines.

Enough already–oh, and what tax incentives does big oil get with regards to ethanol? I’m pretty sure there are none…

I found a Canadian web site that has mileage data for the Prius in different temps. See http://www.hybridexperience.ca/Toyota_Prius_Fuel_Performance.htm#difftemp

Does anybody have any data on the difference between day and night driving?

This chart makes perfectly good sense. For many of the reasons stated above such as more resistance, lower pressures, time to warm up the engine, etc. I do not own a Prius, but I have a 2008 VW GTI with the fuel readout just like a Prius. On days in the 20’s and low 30’s, I will average 29-30mpg on my trip to work. On days when the temperature is in the 50’s or 60’s, I will average between 34-36mpg. This is a huge difference (about 15%-20%). So it’s not just the effect on your prius.
Also, the Canadian site makes sense because I notice the longer I run my car in cold weather and things loosen/warm up, the better mpg I get.
Another factor to condsider is how bad E10 kills fuel mileage. Most fuels are made with up to 10% ethanol. Ethanol only has 2/3 the power of gas, so E10 can easily cost you 2-3 mpg in addition to the losses that winter fuel blends give.

[...] drop-off in mileage - off course there is more to give when your average efficiency is 45MPG. Down from 45 to 32 MPG seems to be a common ‘plaint. Not that far different from my experience with a robust 4WD vehicle. If you were referring to [...]