Why does it get worse MPG in cold weather?

Once you plug a vehicle into a wall socket (even the engine block heater) it becomes meaningless and/or dishonest to quote MPG.

 

Every vehicle I have ever owned over 25 years has suffered a MPG loss in winter driving. As an extreme example, the 2000 GMC Sierra I used to own I could expect around 13 MPG in city driving in early summer without A/C on.

In winter city driving at -40, I was lucky to get 6 MPG.

My parents have a 2003 Buick LeSabre. When operated in temps of around +15 C and no A/C, that car will usually get 20 MPG in city driving. In winter at -40, it will consistently get 14 MPG.

Last winter, during the nasty -40 stuff, my Prius claimed around 9.8 l/100km or 28 MPG. In temps of +12 to +20 C, no A/C, I could easily average 4 l/100km in city driving, or 72 MPG.

This winter has been much milder, and I'm also using a winter front. So far I've been able to keep it around 5.3 l/100km or 53 MPG Imperial gallon.

 

I'm not boastful about the MPG number.
I'm commenting against Ken Cooper's comment about battery efficiency.

Ken@Japan

 

C4 --

I come up with smaller tyres overestimating MPG. The fuel consumption stays the same between the two cars that have travelled the same distance, but the smaller diameter tyre has revolved more times. Since the car computes distance as wheel revolutions * expected tyre diameter, there is a falsely high distance traveled calculated.

 

I disagree. A block heater only helps negate extremely cold outdoor temps. In warmer temps it has little effect. That's why for modern cars, especially the Prius, they're only used in the coldest climates, like Canada.

It's completely different from a plug in vehicle. None of the energy from the heater helps to propel the car, therefore it is not part of the MPG calculation. It's no more dishonest than a person in Florida reporting better MPGs than a person in Minnesota.

 

Kiloarn is like a wolfhound with a bone; she will only be happy when the denominator takes all energy use into account. Since the engine block heater used energy that is not being counted, she balks.

Most other people understand that the G(allon) in mpg refers to petrol consumption only, and is not synonomous with total energy expenditure.

 

I'll assume you are complimenting me instead of namecalling and continue challenging claims that, through my mechanical engineering training, I know to be inaccurate and/or disingenuous.

I have never disputed that, after the engine is warm, a car's efficiency is typically increased nor have I asserted that EBHs are not cost effective.
They probably are, although I haven't yet seen compelling sources cited.

 

Kiloran, not meant to be insulting. I'm sorry it came off that way. Perhaps a better description would be 'a tiger waiting to pounce' ?

 

No offense taken.
It's helpful to have a thick skin when posting on a public forum.
I appreciate your perspectives.

 

I've been calculating based on fill-ups and I have compensated for the mileage difference due to the tires, and I really don't see any more or less difference than normal between my calculations and the MFD, and thinking a bit more about the situation, I think the MFD reading would likely not change noticably:

Firstly, I think that the assumption of fuel consumption staying the same for the same distance travelled is incorrect.. Yes, the wheels have to spin more times to travel the same real distance, but so does the engine, therefore the engine will end up having to burn *more* gas- you'll need more revs to accelerate, more revs to travel a fixed distance.. Additionally, the speedometer over-estimates your speed- this can have some benefit to MPG as you're actually driving slower than you think, but the engine also runs commensurately longer to complete the trip.. Things tend to balance themselves out and as long as you're staying within a few percent of the nominal size of the tires, any error due to tire size would probably be lost in the normal MFD reading variations..

In any case, even if your assumption that fuel consumption stays the same, by going with larger diameter tires, the MFD would be underestimating mileage, which still reinforces my original point- I've found a situation where I'm actually getting *better* mileage in the winter than I got in the summer: a change in tire diameter and width plus better gas than I normally use seem to be the factors involved. I also tend to take fewer short trips in the winter which would definitely have an impact too, but it's hard to gauge exactly what contribution to MPG this is really making... Suffice it to say that I'm extremely happy with the situation and look forward to staying with the Shell gas in the summer and perhaps even changing my summer tires to a brand/model in the larger diameter, narrower width to see if I can improve summer mileage by the same degree..

 

it doesnt matter what size the tires are... you still only use one reference when determining elasped distance and that is the odometer. if your tires are larger in diameter, you will actually go farther, your mileage via the odometer will be under-reported and your mpg's will go down.

also the larger the diameter of the tires, the more torque that is required to get the car moving. that will use more gas.

however, its not the reason for the difference since the tire size difference would have to be much greater than the car could handle.