In recent months, a handful of small gas cars like the Chevy Cruze Eco, Ford Fiesta, and Hyundai Elantra have hit the market as affordable alternatives for drivers seeking hybrid-like fuel economy without the hybrid price premium. Advertisements from Ford, Chevy, and Hyundai have proudly attached the “40 MPG” badge-of-honor to these models in campaigns that are often geared especially to reach a younger, more urban driving demographic. In truth though, there are currently absolutely zero gasoline-only vehicles available in the United States offering EPA-rated combined fuel economy reaching 40 mpg.

So are the carmakers lying? Not really, but the discrepancy between the numbers seen in the ads and the real-world efficiency of those small gas-powered cars reveals that some auto companies are only telling a half-truth.

If you look closely, the 40-mpg claims made in commercials for cars like the Cruze Eco and Ford Fiesta, refer only to highway driving—which is far less taxing on gas cars than the city. The full truth is that the Chevy Cruze Eco for instance, gets 42 mpg on the highway but just 28 mpg in the city, for a combined rating of 33 mpg.

For a number of reasons—such as the use of electric power for low-speed driving and the use of regenerative braking to recapture energy lost while coming to a stop—most hybrids achieve better efficiency in the city than they do on the open road. The Toyota Prius for example, gets a 51 mpg / 48 mpg split, for a combined rating of 50 mpg.

With more than 80 percent of the U.S. population residing in cities or suburbs, most drivers find that the real-world efficiency of their car more closely resembles its city and/or its combined ratings than its highway number. And as gas prices rise, that difference is becoming significantly more valuable.

The Chevy Cruze Eco may start at about $5,000 less than the Toyota Prius, but even if gasoline prices remain at current levels, the average Prius projects to make up its price premium over the Cruze Eco in a little more than seven years. If the price of gasoline were to rise to $4 per gallon, that period drops to six years. Depending on your budget, how much you drive your car, and what kinds of conditions you typically drive in, this may or may not make the Prius or Lexus CT200h—two of only four vehicles that average above 40 mpg combined—a better value for you.

It's an undeniable accomplishment—and a sign that the automotive industry has finally become serious about fuel economy—that there are so many 30-plus mpg models available today for less than $20,000. Still, for drivers in search of truly superior efficiency—barring plug-in models—you still can't beat a good ol' hybrid.

MPG Rating Chart

Discrepancies behind advertised fuel economy