We have had our '05 Prius since late September. We live in the Northern part of Michigan's lower peninsula (winters are cold here).

Our driving is mostly short trips and we have a few hills here. We were somewhat disappointed with mileage through the winter averaging about 42 or 43 mpg.

We took our first long trip with a lot of freeway driving beginning Easter Sunday. First to Deerfield, ILL. (North of Chicago) then Lexington, Ky and on to Pinehurst N.C., about 2500 miles overall. We averaged 53 miles per hour consistently. Of course the weather was much warmer the whole time but it was a different kind of driving than we had been doing.

But we have been back in Michigan for 2 weeks and now the weather is much warmer (75 today) and we are getting over 53 miles per hour. It has to be the warmer weather. I am wondering why.

My dealer doesn't seem to know as much about my car as I do. I am guessing the battery is much more efficient when it is warm. Anybody know for sure or how one poses a question to the Toyoto tech people?

Very happy owners.

HSD is extremely sensitive to temperature fluctuations. Cold weather means longer warmup times and often heat to keep the interior warm. Larger demands on the engine mean the engine may not be able to shut off, effectively transforming HSD into a mild hybrid system instead of a full hybrid system. When the temperature is moderately warm, the engine shuts off more often and allows HSD to maximize efficiency. You'll see a similar plunge in mileage at temperatures over 90 degrees F, as air conditioning demands force the engine once again to remain on to keep the battery charged.

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Warm air for the intake helps mpg.
Warmer battery helps MPG.
Warm ICE improves speed of warm up.
Warm temps help keep the coolant in the thermos warmer.
Warm weather means summer fuel = higher BTU/gallon = less fuel consumption for same mileage.
Warm weather means less dense air to push the car through.

Warm is good!!

You'll actually probably continue to notice mpg improvement even to temps above 90 or 100 degrees if you use the A/C only in moderation...enough to keep the battery cool and you comfortable, but not cold enought to hang meat in the cabin!

Take a look at the graph of my lifetime mileage here in the mileage database (click the thing in my sig) and you'll see what's occured in warm weather vs cold to my mpgs over the life of the car.

Evan E. Fusco, MD
'04 Prius/BC moded to the T
'06 Highlander Hybrid Limited AWD w/NAV

The only thing I can think of that's better than warm weather and a Prius is sitting around with a group of SUV owners talking about how infrequently and inexpensively I refuel.

Oh yeah, and sipping a chilled adult beverage.

Getting the catalitic (sp) converter warm also takes a lot longer in the winter.

My for best fuel economy weather is just after a rain, so the humidity is near 100%, but the roads are relatively dry and the temperature is about 70F.

Water injectors were common when most vehicles used carburators, but those occasional muggy days lead me to think they still might be useful.

61.5mpg lifetime - 82mpg in recent months

Best Run >
www.cleanmpg.com

"fanatic" is what the lazy call the dedicated

With my car, so far in the winter, I pulled 34-38mpg in the winter (depending on how much ground slush) and so far, at ~13000mi, I am averaging 44-46mpg this spring.

I've recorded every tankful of my '03 for over two years, and I've run a regression of mpg vs. temperature. R^2 = 0.33, which means that one-third of the variation in mpg from tankful to tankful is explained by temperture alone. Average mpg at 20 F is 36; average mpg at 80 F is 47. Our overall average year-round is 42.

[quote=efusco]Warm air for the intake helps mpg.
Warmer battery helps MPG.
Warm ICE improves speed of warm up.
Warm temps help keep the coolant in the thermos warmer.
Warm weather means summer fuel = higher BTU/gallon = less fuel consumption for same mileage.
Warm weather means less dense air to push the car through.

Warm is good!!

Yes..BUT:
warm intake air: bad...less dense...fewer moles of O2 to oxidize the fuel. Large diesels are intercooled after the turbocharger to help correct this. gas turbines are also more efficient with cool air for the same reason.

conversely, less dense air IS easier to puch through, as you note. So its a balancing act.

Interesting point on the summer fuel blend...

I suspect the "high humidity = high MPG" comes from the heat capacity of the water keeping the intake air cooler- the H20 absorbs some of the heat that would normally be heating the air and would make it less dense.