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Discussion Starter #1
Since winter versus summer mileage is a regular topic on this forum, I’d like to submit some data from the two-and-a-half winters and two summers I’ve had my Prius.

• The tank has been filled 12 times in January, February or March: 4507 miles ÷ 93.5 gallons = 48.2 mpg. (50.7 on the mfd)

• The tank has been filled 10 times in July, August or September: 4669 miles ÷ 86.9 gallons = 53.7 mpg. (56.3 on the mfd)

So winter mpg is down by 5.5 mpg, or a little over 10 percent. That’s less than I expected.

Colorado’s winter gasoline is 10 percent ethanol, and ethanol has only two-thirds the energy of gasoline, so one should expect a mpg reduction of about three percent in the winter. Ninety-seven percent of 53.7 is 52.1, so the other 3.9 mpg reduction is due to the significantly longer warm-up cycles, the engine working harder in colder weather, and heater use. (Of course, the reduction is actually more, since in the summer neither my wife nor I are sufficiently stoic to entirely avoid the air conditioner.)

The funny thing is that the winter driving has actually cost me less per mile than the summer driving, because the summer mpg increase hasn’t been enough to offset the summer gas price increase. The total winter gasoline cost was $202.36 at $2.16 a gallon, which works out to 4.49 cents per mile; the summer cost was $237.05 at $2.73 a gallon, or 5.08 cents per mile.
 

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I'll go ahead and answer that one for you. Better milage during the day. It's because of the sun's radiation. It heats up the car a bit faster, even at the same temperature. I will say that your milage depends on more than that. I took all conditions into account, like traffic. So the only difference was the sunlight. At night I usually get about the same MPG, or sometimes a bit more, even if it's a bit colder. At night I don't have to worry about traffic as much, and most of the stoplights are either going to always be green on approach or red on approach so I don't have to guess when to coast over a hill to the light or not.
 

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Keep in mind as well that now, according to the EPA, even summer gas will have ethanol in it. No more MTBE or Techron.

Interesting calculation. Good post.
 

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Alex, it all depends on the location and season. Usually nights give me better mileage. For one less traffic helps me keep moving. Two that solar radiation actually hurts me most times, since it takes AC or at least heavy fan to cool down the cabin.
 

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Discussion Starter #6
Alas, my database doesn't break things down by day and night! :) But my impression is that it's basically a wash in this climate - the nighttime gain from fewer stops is roughly cancelled out by the nighttime loss from lower temperatures. The key to great mileage in town is to get the car fully warmed up as quickly as possible, but the thin, dry Colorado air cools down fast once the sun sets. (I'm not complaining, btw - this is great in the summer...)
 

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I know one thing, speaking from a flying point of view...

Daytime air is less dense than cooler night air is. I can't help but think that it makes a difference to a Prius pushing through the air mass in the same way it makes a difference to an aircraft.
 
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