If you are an automaker launching a new, fuel-efficient car, claiming 40 mpg (EPA highway fuel economy) is like the new black. Basically everyone’s doing it. But, will consumers actually achieve that magic number in the real world? To find out, we compared the highway fuel economy in Consumer Reports’ tests to the EPA numbers for cars claiming over 40 mpg on the highway.

A few words on our tests: Consumer Reports measures the fuel economy of every car we purchase and test. We don’t test preproduction models. To get our numbers, we physically measure fuel economy with a precision fuel meter that we put into each car’s fuel line; we’re not going by the amount of gas pumped into the car at a gas pump or the onboard computer.

Our highway fuel economy numbers are measured using two different drivers, driving on a stretch of highway at 65 mph, going in both directions to mitigate wind and roadway slope effects. Wind speed must be below a nominal amount and results are temperature corrected. Since we test in all seasons, the air conditioning is shut off for consistency.

These test results are accurate, real-world figures and they are both repeatable and comparable.

Below are the numbers for all of the gasoline- and diesel-powered cars we’ve tested that claim over 40 mpg highway in the EPA test cycle.

Most cars that claim 40 mpg on the highway can indeed deliver that or beat it, assuming the driving conditions are right. Some cars, like most of the Volkswagen diesels that we have tested, as well as the hybrid Toyota Prius and Lexus CT 200h, can beat the magic number by large margin.

Why the difference? EPA fuel economy numbers are measured using preproduction vehicles, measured in corporate development facilities. (The EPA conducts compliance check tests of approximately 10-15 percent of tested vehicles.) The EPA highway number includes a combination of varying speeds for an average of 48 mph, including a high-speed cycle that reaches 80 mph. The results are then calculated with a multiplier. In other words, they provide a level basis for comparing one vehicle to another, but that number can be hard to relate to the real world.

We’ve also found that some variables can affect the fuel economy of a specific vehicle in the real world but have less effect for a more generic vehicle in laboratory tests. Vehicle weight can be affected by optional equipment, like adding a sunroof, and rolling resistance can be affected by choosing optional tires.

It’s important to note that achieving any fuel economy number depends strongly on how you drive. Getting off the highway and dealing with intersections will reduce economy, an important factor given that very few drivers can drive solely on the highway. Driving faster, especially at highway speeds, can significantly reduce fuel economy; likewise driving slower improves it. Using air conditioning has an effect, as does carrying passengers or mounting a roof rack.

For more on fuel economy, see our special section.