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Advertised MPG: Almost Meaningless Today

With the cost of a gallon of gasoline today near or in excess of $3.50, consumers are paying more attention to the EPA fuel-economy ratings.  Manufacturers are required by law to post their vehicles’ fuel-economy ratings, as certified by the federal environmental protection agency (EPA) on the window stickers of most every new vehicle sold in the U.S. – the only exception being vehicles with a gross weight of 8,500 pounds.  Many consumers rightly complain that the EPA’s numbers are substantially higher than what an average owner might expect to experience in real world driving.

A lot has to do with the way new cars and trucks are evaluated for their energy consumption.  While it would seem logical to determine a vehicle’s fuel economy simply by filling up the tank, driving it on the road or a test track for a set number of city or highway miles, refilling the tank, and dividing the number of miles driven by the number of gallons consumed, this is not how the experts do it.

In fact, tested vehicles don’t even reach the pavement at all!  Rather, a car or truck’s fuel economy is measured under rigidly controlled circumstances in a laboratory using a standardized test that is mandated by federal law.  Automakers actually do their own fuel economy testing and submit the results to the EPA, which reviews the data and confirms about 10 to 15% of the ratings.  Each new car or truck is tested on what is called a dynamometer, which is like a large treadmill.  While the engine and transmission drive the wheels, the vehicle never actually moves, just the rollers upon which the wheels are placed.  A professional driver runs the vehicle through two standardized driving schedules, one each to simulate city and highway driving conditions, and ensures he or she is maintaining the mandated pace via a real-time computer display.

The “city” program is designed to replicate an urban rush hour driving experience in which the vehicle is started with the engine cold and is driven in stop and go traffic with frequent idling.  The car or truck is driven for 11 miles and makes 23 stops over the course of 31 minutes, with an average speed of 20 mph and a top speed of 56 mph.  The “highway” program, on the other hand, is created to emulate rural and interstate freeway driving with a warmed up engine, making no stops.  The vehicle is driven for 10 miles over a period of 12.5 minutes with an average speed of 48 mph and a top speed of 60 mph.  Both fuel economy tests are performed with the vehicle’s air conditioning and other accessories turned off.

There exists a host of other physical and personal factors that contribute to the differences between a vehicle’s rated and realized energy consumption.  Cars and trucks used in the EPA testing are broken in and are in top mechanical shape.  New vehicles don’t usually attain their top mileage until they have been driven about 3,000 to 5,000 miles.  Even relatively minor upkeep factors like having incorrect air pressure can cause minor disparities in fuel economy.  Depending upon where you live, the particular blend of gasoline sold in your area at a given time of the year may have more or less energy content which in turn results in better or worse fuel economy.

Also, the cars and trucks subjected to fuel economy testing are “driven” without a full complement of passengers, cargo, and options aboard.  Similarly, the vehicles are tested without the air conditioning and other electrical accessories in use.  Other physical factors like trip length, traffic conditions, terrain, temperature, and the weather all affect your mileage.  Likewise, installing exterior accessories like roof racks and cargo carriers that hamper a vehicle’s aerodynamics will take a toll at the pump.

About the only thing that can be said for the EPA testing is that it is still valid as a source of comparison when you shop for a new vehicle.  The EPA is currently developing new, more accurate ways of testing vehicle fuel economy which will take into consideration real world driving factors like aggressive acceleration, hot and cold weather driving conditions, and the use of air conditioning while driving. But for now, the testing in relation to “real world” driving, it is all but meaningless.

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