The common complaint among hybrid owners is that their real-world fuel economy doesn’t match the official estimate given by the Environmental Protection Agency (EPA)—by as much as 20 percent. Why the gap?There are a few reasons, according to an article posted July 11 at TheCarConnection.com. First, an explanation of the EPA test: Models are tested on a dynamometer, a treadmill for cars. A professional driver runs a “city” and “highway” program. In the city program, 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. In the highway program, 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. A hose on the tailpipe collects engine exhaust. The amount of carbon from the exhaust system is measured to calculate the amount of fuel burned. The EPA claims this is more accurate than using a fuel gauge to physically measure the amount of gasoline burned. Still, the final test figures are adjusted downward, by 10 percent for city driving and 22 percent in highway mileage, to help reflect the differences between what happens in a lab and out on an actual road.Basing fuel economy upon the amount of carbon exhaust that's emitted by a vehicle's tailpipe automatically favors gas/electric-powered vehicles. Since some of a hybrid's power comes from an electric motor that automatically produces zero emissions, these figures tend to skew higher than simple miles-driven/gallons consumed computations would otherwise indicate, the article said.Other general factors:--The vehicles are tested without the air conditioning and other electrical accessories in use.--EPA's test vehicles are in top mechanical shape. Ill-maintained vehicles will consume more gas than those that are in perfect condition. --Different blends of gasoline may have more or less energy content in different parts of the country, which in turn results in better or worse fuel economy. --Physical factors like trip length, traffic conditions, terrain, temperature, and the weather all affect mileage.--Vehicle accessories (roof racks and cargo carriers) hamper aerodynamics and thus mileage. --Lead-footed acceleration, heavy braking, high-speed driving, excessive idling, towing, and engaging four-wheel-drive also negatively impact mileage, by as much as 33 percent.