Automakers 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 exception is for vehicles having gross-vehicle-weight ratings over 8,500 pounds, which include heavy-duty pickups and the largest SUVs.

The posted information lists the miles per gallon estimate for city driving and for highway driving, and also estimates the fuel economy range that most drivers achieve with that particular model.

For a listing of EPA estimates for all vehicles covered by the program, visit http://www.fueleconomy.gov. As most of us can attest, these "official" ratings rarely reflect our own real-world driving experience. Fuel economy is not a fixed number. Depending on what, how, and where you drive, the differences can be pronounced. Your vehicle's fuel economy will almost always differ from EPA's fuel economy rating.

The EPA ratings estimate the mpg a "typical" driver should get under "typical" city and highway conditions. However, most drivers and driving environments aren't typical, and the factors that affect fuel economy can vary significantly.

EPA vs. Real-World MPG

Here's a sampling of the EPA fuel-economy estimates for a variety of vehicles, and the actual miles-per-gallon averages observed by Consumer Guide® during its road-test program. This road-test program subjects vehicles to a mix of city and highway driving by at least four road-test editors. The cars are tested in the Chicago area and in Southern California, and usually accumulate about 300 miles during the test period. (Note: man. means manual transmission; auto. means automatic transmission; awd means all-wheel drive.)

How New Vehicles Are Tested

Why do the EPA's mileage-per-gallon numbers almost always seem to be higher than the real-world mpg numbers from Consumer Guide®'s testing? It has a lot 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 by simply 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 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's mandated by federal law. Automakers actually do their own testing and submit the results to the EPA, which reviews the data and confirms about 10 to 15 percent of the ratings itself at the National Vehicles and Fuel Emissions Laboratory.

Each model is tested on what's called a dynamometer, which is like a treadmill for cars. 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.

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 is created to emulate rural and interstate freeway driving with a warmed-up engine, making no stops (both of which ensure maximum fuel economy). 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 tests are performed with the vehicle's air conditioning and other accessories off.

Throughout the test a hose is connected to the vehicle's tailpipe and collects the engine's exhaust. It's the amount of carbon that's present in what's spewed from the exhaust system that's 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 that's being 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.

The EPA and Hybrids

The gap between official and experienced fuel economy can be even wider for owners of gas/electric hybrid-powered vehicles. Most experts feel the EPA's ratings for hybrid vehicles tend to be overstated by a factor of at least 20 percent. This discrepancy can be wider yet if a motorist drives primarily on the highway, where hybrids tend to be less efficient than in stop-and-go city driving conditions (during which the electric motor shoulders more of the effort).

Ironically, the results of ongoing operating tests conducted by the EPA of a dozen hybrid cars in its own fleet significantly contradict their posted fuel-economy ratings. The best the EPA's fleet could muster was a cumulative average of 37.7 mpg for the 2004 Honda Civic, 45.7 mpg for the Honda Insight, and 44.8 mpg for the Toyota Prius. While this is certainly admirable fuel economy, it's still far below the cars' EPA ratings that run as high as 51, 66, and 60 mpg, respectively, for the model years tested.

Why do such discrepancies tend to be more pronounced for hybrids? Experts say it's because basing fuel economy upon the amount of tailpipe exhaust 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.

Discrepancies Beyond the Lab

In addition to the testing methods used to determine the EPA's ratings, a host of other physical and personal factors contribute to the differences between a vehicle's rated and realized energy consumption. For starters, cars and trucks used for evaluation in the EPA's tests are broken in and are in top mechanical shape.

Also, the cars and trucks subjected to fuel economy testing are "driven" without a full complement of passengers and cargo. Similarly, the vehicles are tested without the air conditioning and other electrical accessories in use.

While the EPA's fuel-economy estimates may not be a completely accurate prediction of the kind of mileage you'll register during your daily commute, it's still valid as a source of comparison when you shop for a new vehicle.

Are you thinking about buying an SUV? Or perhaps you'd prefer one of those new, fuel-efficient hybrids. Can using diesel really save you a few bucks at the tank? In the next section, we'll take a look at all three vehicle types and help you decide if one of them is right for you.