The new (Dec. 2012) CR tests the Altima four and six. They show 16/35 for the six and 21/44 for the four.
This is their own loop, "highway" is 65 mph in two directions on a closed loop, not the EPA or any other arrangement.
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.