Hybrid Car – More Fun with Less Gas

Winter Gas Mileage

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Posted by sd100 on February 19, 2006, 7:03 pm
I had the same problem you are noting.  I changed the air cleaner filter and
the mileage went back up to 50+.  Also, make sure your tires, whether they
be snows or not, have the correct air pressure in them.

Posted by Mr. Skeptic on March 4, 2006, 1:01 pm
Recently, we've had some real temperature swings in our area, with
several days in the 30's followed by several days in the 50's, and so
forth. I've noticed the gas mileage always goes down significantly in
the colder weather.

I used to think it was the winter gas mixture, but now I'm thinking it
might be due solely to the termperature. I know colder air is denser
and thus wind resistance is greater, but I wouldn't think that fact
would be very significant at car speeds. So I assume the real culprit
is that combustion is less efficient. But why?

Posted by richard schumacher on March 4, 2006, 4:38 pm

1. It takes more energy to heat cold air than warm air.

2. The engine has to run longer to bring itself and the catalytic
converter up to their correct operating temperatures.

Posted by Bill on March 4, 2006, 6:36 pm

Yes, but these aren't the only two factors.  I think you are both right.
I've observed a reduction in mileage during cold weather after my converter
is hot and after my interior is warm while at a speed that keeps the ICE
running full time.  One trip I take periodically involved a 120 mile round
trip on I29 in the red river valley of North Dakota, the planet's flattest
expanse of land.  In the summer I can lock my car at 55 mph when wind isn't
a factor and achieve a perfectly flat series of five minute bars at 53 mpg.
In the winter the same trip yields a perfectly flat series at 51 mpg.  I've
repeated this several times.  Overall mileage drops more due to the two
factors you mention but I believe the 2 mpg difference doesn't involve these

Posted by richard schumacher on March 5, 2006, 4:06 am

Point (1) above did not refer mostly to heating air inside the passenger
compartment.  A much larger factor is the air used inside the engine
cylinders.  The expanding mixture of heated air and combustion gases
make the car go.  When the engine intake air is colder it takes more
energy to heat it and do the necessary work of moving the pistons.

Yes, winter gas reduces MPGs.  Yes, colder air is denser, and yes, the
Prius burns more gas just to keep itself warm; but the mere fact that
the engine is a *heat* engine means that colder inlet air will require
more fuel to make it produce the same power.

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