One burning question for all vehicles, not just hybrids, is whether they really achieve the Environmental Protection Agency's fuel-economy estimates posted on a new vehicle's window sticker. The answer is…sometimes.

Experience over many years has shown that conservative driving can lead to the city and highway fuel-economy numbers the Environmental Protection Agency tells us we should be able to achieve with a given model. But then, who among us really drives that conservatively?

This is an especially hot topic with hybrid drivers. In fact, it's been widely reported that real-world hybrid fuel economy is lower than official EPA estimates by a substantially wider margin than conventional vehicles. This is no doubt due to the idiosyncrasies of gasoline/electric propulsion and the EPA's laboratory test procedures, which were designed long ago and are in need of updating. In fact, there's substantial pressure on the EPA to revise its testing procedures to reflect real-world driving and generate more accurate fuel-economy figures, not only for hybrids but for all vehicles.

The Union of Concerned Scientists reports the disparity between estimated and actual fuel economy for vehicles in general as high as 30 percent. Additionally, a Consumer Reports study that examined the fuel economy of over 300 model year 2000 to 2006 cars showed highway mileage reasonably close to EPA estimates, but a disparity of 35 percent to 50 percent in city driving figures. Interestingly, while hybrids in the Consumer Reports testing averaged 19 mpg less than EPA city fuel economy estimates, three hybrids still ranked among the study’s top five spots for fuel economy.

This underscores a simple point: While it may be that hybrid vehicles often fail to achieve the ethereal estimated fuel-economy figures on their window stickers, they are still generally achieving the highest fuel economy of any vehicle on the road and are worthy of your consideration.

The EPA's Ancient System

The Environmental Protection Agency's fuel economy tests begin with data submitted by automakers based on their own internal testing conducted under controlled laboratory conditions using a standardized test procedure specified by federal law. EPA reviews the results and confirms about 10 percent to 15 percent of these with testing at its own laboratory.

In either case, lab testing is done on a dynamometer following a standardized routine that simulates city and highway driving conditions -- or so it seems. This protocol does not reflect today’s driving patterns because testing assumes an average speed of 48 mph and a top speed of 60 mph, little congestion, gentle acceleration and braking, no use of mpg-robbing air conditioning and only moderate ambient temperatures.

Ultimately, the EPA determines fuel economy numbers by measuring the amount of carbon in each vehicle's exhaust that’s captured during testing. The EPA says this is a more precise indicator of fuel consumption than a fuel gauge. As an acknowledgement that its testing protocol reflects ideal conditions and to avoid overestimating real-world mileage, the EPA began reducing city fuel-economy results by 10 percent and highway results by 22 percent in the 1980s. No additional adjustments have been instituted, or testing procedures modified, to reflect significant changes in motor-vehicle technology since then.

Until those procedures do change, it's best to keep in mind that disparities will exist and that differences in operating conditions, climate and driving habits can influence actual fuel economy considerably.

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