[mrv]

Chances are that you don't drive like
the outdated EPA mileage tests...

http://www.fueleconomy.gov/feg/info.shtml#estimates

<blockquote>
How are fuel economy estimates obtained?

The fuel economy estimates are based on results of tests required by
the U.S. Environmental Protection Agency (EPA). These tests are used
to certify that vehicles meet the Federal emissions and fuel economy
standards. Manufacturers test pre-production prototypes of the new
vehicle models and submit the test results to EPA. . EPA re-tests
about 10% of the tested vehicles to confirm manufacturer's results in
EPA's lab. The vehicles are driven by a professional driver under
controlled laboratory conditions, on an instrument similar to a
treadmill. These procedures ensure that each vehicle is tested under
identical conditions; therefore, the results can be compared with
confidence.

There are two different fuel economy estimates for each vehicle in
the Fuel Economy Guide, one for city driving and one for highway
driving. To generate these two estimates, separate tests are used to
represent typical everyday driving in a city and in a rural setting.
Two kinds of engine starts are used: the cold start, which is similar
to starting a car in the morning after it has been parked all night;
and the hot start, similar to restarting a vehicle after it has been
warmed up, driven, and stopped for a short time.

The test used to determine the city fuel economy estimate simulates
an 11-mile, stop-and-go trip with an average speed of 20 miles per
hour (mph). The trip takes 31 minutes and has 23 stops. About 18
percent of the time is spent idling, as in waiting at traffic lights
or in rush hour traffic. The maximum speed is 56 mph. The engine is
initially started after being parked overnight. Vehicles are tested
at 68 F to 86 F ambient temperature.

The test to determine the highway fuel economy estimate represents a
mixture of "non-city" driving. Segments corresponding to different
kinds of rural roads and interstate highways are included. The test
simulates a 10-mile trip and averages 48 mph. The maximum speed is 60
mph. The test is run with the engine warmed up and has little idling
time and no stops (except at the end of the test).

NOTE: To make the numbers in the Fuel Economy Guide more useful for
consumers, EPA adjusts these laboratory test results to account for
the difference between controlled laboratory conditions and actual
driving on the road. The laboratory fuel economy results are adjusted
downward to arrive at the estimates in the Fuel Economy Guide and on
the labels seen on new cars, light trucks, and vans. The city
estimate is lowered by 10% and the highway estimate by 22% from the
laboratory test results. Experience has proven that these adjustments
make the mileage estimates in the Fuel Economy Guide correspond more
closely to the actual fuel economy realized by the average driver.
</blockquote>


The EPA test cycles were developed more than 30 years ago, and based
on the typical driving patterns of that time. "Correction factors"
have been added to lower the values to more "realistic" values, based
on the last US driving study performed, but that was back in 1985.
Many reports have shown that American's driving patterns have changed
in that time period (longer commutes, increased urbanization, more
congestion, higher speed limits, etc.)

A good paper (cites its references) that I found on the subject is
here:
http://www.bluewaternetwork.org/reports ... ehood2.pdf

 

[tjl]

Chances are that you don't drive like
the outdated EPA mileage tests...

The EPA test cycles were developed more than 30 years ago, and based
on the typical driving patterns of that time. "Correction factors"
have been added to lower the values to more "realistic" values, based
on the last US driving study performed, but that was back in 1985.
Many reports have shown that American's driving patterns have changed
in that time period (longer commutes, increased urbanization, more
congestion, higher speed limits, etc.)

A good paper (cites its references) that I found on the subject is
here:
http://www.bluewaternetwork.org/reports ... ehood2.pdf

On the other hand, people who drive for fuel economy routinely beat the EPA fuel economy estimates. Some people on http://www.insightcentral.net get more than 75mpg (while the Insight is rated for at most 70mpg on the EPA highway test), with one claiming 90mpg (though the 90mpg sample is all highway at 50mph or 55mph). But it isn't limited to Insights; people on http://www.tdiclub.com routinely describe getting over 50mpg in cars with EPA highway ratings of only 49mpg.

It may be true that the "typical" American driver gets much worse than the EPA fuel economy because s/he has fuel wasting habits like:

* Racing to red lights and then slamming on the brakes.
* Tailgating, which forces them to constantly adjust speed in reaction to the speed changes of the vehicle in front.
* Not maintaining his/her car properly.
* Driving on underinflated tires (which are also unsafe).
* Doing idle warmup (in most conditions, not needed and is actually worse for the car because it warms up more slowly).

But I'd expect that someone cares about fuel economy enough to buy, or consider buying, a Prius or other fuel efficient car will avoid doing the above.